Mednotes

L13 Metabolism Nitrogen balance and shuttling

Properties2
tagsmetabolism, lecture
aliasesL13 Metabolism Nitrogen balance and shuttling

31 min read

Overview

This lecture series covers normal nitrogen metabolism, the clinical problems that arise when it fails, and inborn errors of metabolism. It follows nitrogen from the amino acid pool (fed by dietary and body protein) through the concept of nitrogen balance, the separate fates of amino acid carbon skeletons and amino groups, the shuttling of excess amino groups to the liver as alanine and glutamine, their conversion to urea via the urea cycle, and finally the excretion of nitrogen compounds in urine. It closes with inborn errors of metabolism, using phenylketonuria as the worked example. The tutorial covers a defect in nitrogen metabolism and biochemical data as a diagnostic tool, and the laboratory covers urine analysis as an indicator of nitrogen metabolism status; material from lectures, tutorial or laboratory can all appear in the end-of-year exam.

Framing clinical case

Robert Griffin, a 38-year-old NZ European male with active Crohn’s disease, has had multiple small bowel resections complicated by bowel leaks and fistulas. Ongoing inflammation has caused new fistula complications and slow healing, so he was prescribed three months of bowel rest with total parenteral nutrition (TPN), starting in hospital and transitioning to home. At the start of TPN he was in negative nitrogen balance; adding glutamine to the TPN markedly improved his nitrogen balance.

The exam questions built on this case, with their marking-scheme level:

  • Discuss the concept of nitrogen balance (CONCEPT).
  • Explain the role of glutamine and alanine in maintaining nitrogen balance and eliminating nitrogen (DETAIL).
  • Why would Mr Griffin have been in negative nitrogen balance before TPN, and why did TPN improve his nitrogen balance status? (CLINICAL APPLICATION and INTEGRATION).
  • Due to his underlying illness, why might Mr Griffin be at increased risk for hyperglycaemia? (CLINICAL APPLICATION and INTEGRATION).

The amino acid pool: sources and fates

The amino acid pool is the central hub of nitrogen metabolism.

Inputs:

  • Ingested protein.
  • Body proteins, which are in equilibrium with the pool (continuous turnover in both directions).

Fates out of the pool:

  • Excess nitrogen leaves as NH3 and is converted to urea.
  • Carbon skeletons feed non-essential amino acid synthesis, and glycolysis/TCA cycle intermediates, which exchange with glucose and glycogen stores.
  • Glycolysis/TCA intermediates feed acetyl CoA, which produces fatty acids and other lipids.
  • Acetyl CoA, together with the amino acid pool, yields energy plus CO2 and H2O.
  • The pool supplies purines, pyrimidines, porphyrins, thyroxine, neurotransmitters, phospholipids and coenzymes.

Protein turnover: about 300 g of protein are degraded and synthesised per day in the whole body, which is 3% replacement per day. By organ (percentage replacement per day, total protein synthesis in g/day, percentage contribution to whole-body synthesis):

Organ% replacement/dayg/day% of whole body
Whole body3%300(100)
Skeletal muscle2%12041%
Liver10%/7%8025%
Small intestine14%7023%
Large intestine7%83%
Kidneys5%31%
Heart2%10.4%

Key points: muscle contributes the largest share of whole-body protein synthesis by mass, but the small intestine has the highest fractional turnover rate (14% per day).

Essential and non-essential amino acids

Both classes are equally important; the distinction is only whether the body can make them. Non-essentials can be synthesised, essentials cannot and must come from the diet.

Origins of the non-essential amino acids:

  • Alanine: from pyruvate via transamination.
  • Aspartic acid, asparagine, arginine, glutamic acid, glutamine, proline: from citric acid cycle intermediates.
  • Serine: from 3-phosphoglycerate (glycolysis).
  • Glycine: from serine.
  • Cysteine: from serine, requiring sulfur derived from methionine.
  • Tyrosine: derived from phenylalanine by hydroxylation.

Essential amino acids (mnemonic PVT TIM HALL) with their notes:

  • P phenylalanine: also required in the diet as a precursor of tyrosine.
  • V valine: one of three branched-chain amino acids.
  • T threonine: metabolised like a branched-chain amino acid.
  • T tryptophan: its complex heterocyclic side chain cannot be synthesised in humans.
  • I isoleucine: one of three branched-chain amino acids.
  • M methionine: provides the sulfur for cysteine and acts as a methyl donor; the homocysteine is recycled.
  • H histidine: its heterocyclic side chain cannot be synthesised in humans.
  • A arginine: can be derived from ornithine in the urea cycle in amounts sufficient for adults, but growing animals require it in the diet.
  • L leucine: a pure ketogenic amino acid.
  • L lysine: neither of the nitrogens of lysine can undergo transamination.

Note that cysteine and tyrosine are conditionally dependent on essential amino acids (methionine and phenylalanine respectively). Selective use of essential versus non-essential amino acids can be beneficial in the treatment of nitrogen metabolism disorders.

Amino acids as precursors of other molecules

From tyrosine:

  • Catecholamines: tyrosine to 3,4-dihydroxyphenylethylamine (dopamine) to norepinephrine to epinephrine.
  • Thyroid hormones: triiodothyronine and thyroxine.
  • Melanin.

From other amino acids:

  • Tryptophan to 5-hydroxytryptophan to serotonin (5-hydroxytryptamine), with loss of CO2.
  • Tryptophan to melatonin.
  • Histidine to histamine.
  • Arginine to nitric oxide (NO).

Metabolically important products and the amino acids that build them:

  • Creatine/phosphocreatine (phosphocreatine yields creatinine + Pi): from arginine, glycine and methionine.
  • Carnitine: from lysine and methionine.
  • Glutathione, existing as reduced GSH and oxidised GSSG: from glutamate, cysteine and glycine.

Plasma amino acid reference ranges

All values µmol/L, given for age under 4 months and over 4 months:

Amino acid<4m>4m
Taurine20-25616-142
Aspartic acid5-1000-173
Threonine114-33681-217
Serine94-32488-288
Glutamine530-960544-836
Proline107-435185-285
Glycine224-515100-390
Alanine236-675176-480
Cystine35-9220-100
Valine80-370100-330
Methionine10-965-80
Iso-leucine27-10523-98
Leucine46-23060-220
Tyrosine42-19645-100
Phenylalanine42-18221-133
Ornithine49-21425-105
Lysine114-316110-284
Histidine49-19520-220
Arginine22-15532-130
Citrulline5-6320-70
Glutamic acid20-1425-130

Glutamine has by far the highest plasma concentration of any amino acid.

Nitrogen balance

Why balance matters:

  • Humans have no dedicated nitrogen store that can be drawn on to make nitrogen-containing molecules.
  • Humans cannot avoid losing some nitrogen every day, through excretion and loss of cells and biomolecules.
  • Therefore nitrogen must both be replenished (mainly via dietary protein) and excreted when in excess.

Nitrogen balance status depends on a combination of:

  1. Dietary nitrogen intake (cellular material and dairy).
  2. Physiological state, which influences nitrogen retention and excretion.

The three states:

  • In balance: nitrogen intake = nitrogen excretion. Intake is dietary amino acids, nucleotides and so on; excretion is via urine, faeces, hair and skin loss, and perspiration. On the metabolic map, ingested protein and urea output are equal. Balance is maintained even when both intake and excretion are elevated together.
  • Positive: nitrogen intake > nitrogen excretion, associated with an increase in lean mass. There is net flux from the amino acid pool into body proteins, and urea output is reduced relative to intake.
  • Negative: nitrogen intake < nitrogen excretion, associated with a decrease in lean mass. There is net flux from body proteins into the amino acid pool, urea output rises, and intake of protein or energy may be reduced.

The slides posed possible causes of positive and negative balance as a discussion prompt (”???”) without listing answers.

Important

Reduced energy intake, not only reduced protein intake, is shown on the negative-balance diagram as a cause of negative nitrogen balance (‘or decrease energy intake’).

Negative nitrogen balance in injury and trauma

Nitrogen excretion (g per 70 kg per day) rises with the severity of injury. Approximate values from the bar chart, in descending order: burns ~27; injury ~18; cystectomy ~13; sepsis ~12.5; hip replacement ~8; malnourished ~7.5; normal ~4; normal fasted ~2.5.

Phases of the metabolic response to injury:

  1. Anticipation phase (fright, fight or flight): preceding the injury (before time 0 on the timeline).
  2. Ebb phase (shock): hours 0-24 after injury.
  3. Flow phase (catabolic): from about 24 hours after injury to about 3 weeks.
  4. Convalescent (anabolic) phase: months.

Features of the catabolic phase: increased metabolic rate; weight loss; evidence of muscle wasting (negative nitrogen balance).

Contributing factors: metabolic requirements for tissue repair and the immune response; insulin resistance; cytokines and inflammatory mediators (TNF-alpha, IL-2); lack of appetite.

Protein oxidation as a percentage of resting energy expenditure rises with illness: normal ~5%, sepsis ~9%, injury ~12.5%.

Warning

Both bar charts in this section had no numeric labels on the slide; the transcript records the values as read approximately from the chart.

Fates of amino acid carbon skeletons

Once the amino group is removed, the remaining carbon skeleton is classified as glucogenic (enters gluconeogenesis via pyruvate or a TCA intermediate) or ketogenic (enters as acetyl CoA or acetoacetyl CoA, and so can form ketone bodies or lipids but not glucose).

Glucogenic entry points:

  • To pyruvate: alanine, glycine, cysteine, serine, threonine, tryptophan.
  • To oxaloacetate: aspartate, asparagine.
  • To fumarate: tyrosine, phenylalanine, aspartate.
  • To succinyl CoA: isoleucine, methionine, valine.
  • To alpha-ketoglutarate: glutamine, glutamate, histidine, proline, arginine.

Oxaloacetate, fumarate, succinyl CoA and alpha-ketoglutarate all feed the TCA cycle via citrate, and oxaloacetate/pyruvate feed PEP and then glucose.

Ketogenic entry points:

  • To acetyl CoA: isoleucine, leucine, tryptophan.
  • To acetoacetyl CoA: leucine, lysine, phenylalanine, tyrosine, tryptophan (acetoacetyl CoA and acetyl CoA are interconvertible).
  • Acetyl CoA from the TCA cycle/citrate also feeds lipids.

Several amino acids appear in both lists and are therefore both glucogenic and ketogenic (isoleucine, tryptophan, phenylalanine, tyrosine). Leucine is described on the essential amino acid table as a pure ketogenic amino acid. Lysine appears only among the ketogenic entry points and carries the same asterisk as leucine there, but the slide never defines that marker, so treating lysine as purely ketogenic is an inference from the diagram rather than a taught fact.

Ammonia and urea

Urea:

  • Synthesised in the liver, specifically in periportal hepatocytes.
  • Maintains nitrogen in a soluble, non-toxic form.
  • Transported in blood to the kidney for excretion.
  • Structure: H2N-C(=O)-NH2.

Gut-liver-kidney ammonia handling:

  • The liver runs the urea cycle and exports urea to the kidney for excretion; NH4+/NH3 also move between liver and blood.
  • Urea passes from the blood into the intestine, where urease-producing intestinal bacteria convert urea back to NH3 in the lumen.
  • In the enterocyte, glutaminase converts glutamine (GLN) to glutamate (GLU), releasing NH3, which crosses into the blood as NH3/NH4+.
  • So the gut is itself a source of ammonia entering the portal circulation.

Hepatic encephalopathy and portal-systemic shunting (content shown from a GI physiology/portal hypertension slide):

  • Reduced hepatic detoxification.
  • Toxins, particularly NH3, bypass the liver via shunts (protein-related).
  • Result is cerebral dysfunction and neuropsychiatric symptoms.
  • Pathogenesis: ammonia generated by gut bacteria acting on proteins and urea; normally the liver detoxifies it. In liver failure the liver cannot detoxify ammonia, and/or ammonia does not reach the liver, so systemic levels rise.

Warning

The transcript flags this slide as carrying an “ELM 2” heading, apparently reused from a different module’s lecture on portal hypertension rather than authored for this lecture.

The key nitrogen-handling enzymes

Glutamine synthetase and glutaminase (an opposing pair):

  • Glutamine synthetase: glutamate + NH3 (NH4+) + ATP to glutamine + ADP (+ H2O, Pi). This traps free ammonia.
  • Glutaminase: glutamine + H2O to glutamate + NH3 (NH4+). This releases ammonia.

Transaminations (aminotransferases):

  • General scheme: glutamate + alpha-keto acid is reversibly converted to alpha-ketoglutarate + alpha-amino acid, catalysed by glutamate aminotransferase.
  • The reaction is reversible. (Separately, the learning objectives note the utility of exploiting aminotransferase reactions for treating urea cycle disorders.)

Glutamate dehydrogenase (GDH):

  • Glutamate + NAD(P)+ + H2O is reversibly converted to alpha-ketoglutarate + NH4+ + NAD(P)H.
  • The reaction runs in either direction depending on cellular conditions, so GDH is the point at which amino nitrogen collected onto glutamate is released as free ammonium for urea synthesis, or recaptured.

Together these mean nitrogen from most amino acids can be funnelled by transamination onto alpha-ketoglutarate to form glutamate, then released as NH4+ by GDH or packaged into glutamine by glutamine synthetase.

Shuttling nitrogen to the liver

The liver extracts mainly alanine and glutamine from the circulation, and excess amino groups are released into the circulation mainly as alanine and glutamine. On the arterial-venous difference chart, alanine and glutamine show much the largest uptake by the liver (arterial minus hepatic vein differences of roughly 95-100 µmol/L) and the largest release from muscle (arterial minus femoral vein differences of roughly -65 to -70 µmol/L). All other amino acids (Gly, Lys, Pro, Thr, His, Leu, Val, Arg, Phe, Tyr, Met, Ile, Tau, Orn, AIB, Cit, Cys, Ser) show much smaller values.

The general two-tissue scheme:

  • Most tissues: glutamate + NH4+ + ATP to glutamine + H2O + ADP + Pi, via glutamine synthetase.
  • Liver: glutamine + H2O to glutamate + NH4+, via glutaminase; the NH4+ feeds the urea cycle to make urea.

The inter-organ shuttle across muscle, liver and kidney:

  • Muscle: protein breaks down to amino acids; amino acids transaminate with alpha-ketoglutarate to give glutamate, and with pyruvate to give alanine; the resulting alpha-keto acids are used for energy; glutamate + NH4+ forms glutamine. Alanine and glutamine travel to the liver.
  • Liver: glutamine to glutamate releasing NH4+; alanine transaminates to pyruvate + glutamate, and the pyruvate feeds gluconeogenesis; NH4+ from both routes feeds urea synthesis via the alpha-ketoglutarate/glutamate system; glutamate can also regenerate glutamine.
  • Kidney: glutamine to NH4+ + glutamate; urea is excreted.

So alanine carries both nitrogen and a gluconeogenic carbon skeleton to the liver, while glutamine is a nitrogen carrier to the liver that is also used by the kidney.

Hepatic zonation: periportal and perivenous hepatocytes

Blood enters the lobule from the hepatic artery and portal vein, passes the periportal zone and then the perivenous zone before leaving to the systemic circulation. The two zones handle ammonia differently: the periportal zone runs glutaminase, glutamate dehydrogenase and the urea cycle at speed, and the perivenous zone lowers NH3 before blood leaves the liver.

Periportal hepatocytes:

  • Glutaminase and glutamate dehydrogenase raise intracellular NH3/NH4+.
  • The urea cycle runs here, at speed.
  • Transport detail: Gln enters with Na+ via SNAT3/5; intracellular Gln + H+ gives NH4+ + Glu (glutaminase); Glu gives NH4+ + alpha-ketoglutarate, feeding the urea cycle, and urea is exported. Gln/Ala are also taken up via SNAT2/4 with Na+ and feed gluconeogenesis to glucose. Incoming NH4+ from portal blood drives the urea cycle.

Perivenous hepatocytes:

  • Glutamine synthetase lowers NH3 before blood leaves the liver, acting as a final scavenger of ammonia that escaped the periportal zone.
  • Transport detail: NH4+ and H+ are taken up via SNAT3 with Na+ exported; Glu + NH4+ gives Gln (glutamine synthetase), and Gln is exported; SNAA/SNAT2/4 amino acid exchange also occurs.

The urea cycle

Location and steps (mitochondrion then cytosol), with the numbered enzymes:

  1. Carbamoyl phosphate synthetase (CPS), mitochondrial: 2 ATP + HCO3- + NH3 gives carbamoyl phosphate + 2 ADP + Pi.
  2. Ornithine transcarbamoylase, mitochondrial: carbamoyl phosphate + ornithine gives citrulline + Pi. Citrulline is exported to the cytosol.
  3. Argininosuccinate synthetase, cytosolic: citrulline + aspartate + ATP gives argininosuccinate + AMP + PPi.
  4. Argininosuccinate lyase, cytosolic: argininosuccinate gives arginine + fumarate.
  5. Arginase, cytosolic: arginine + H2O gives ornithine + urea. Urea is released and ornithine re-enters the mitochondrion via a transporter to continue the cycle.

How nitrogen enters the cycle: the two nitrogens of urea come from different sources.

  • One enters as NH3/NH4+ at the CPS step. That ammonium is generated by transamination (amino acid + alpha-ketoglutarate gives alpha-keto acid + glutamate) followed by glutamate dehydrogenase (glutamate + NAD(P)+ gives alpha-ketoglutarate + NH4+ + NAD(P)H).
  • The other enters as the amino group of aspartate at the argininosuccinate synthetase step.

Link to the TCA cycle (the aspartate-argininosuccinate shuttle): the fumarate released at step 4 is hydrated to malate and then oxidised to oxaloacetate with reduction of NAD+ to NADH; oxaloacetate is transaminated back to aspartate using glutamate/alpha-ketoglutarate or other amino acid/alpha-keto acid pairs, regenerating the aspartate the cycle needs.

Energy cost: 2 ATP at the CPS step plus 1 ATP consumed to AMP + PPi at the argininosuccinate synthetase step.

Regulation of the urea cycle

Two major points of regulation:

  1. Concentration of urea cycle enzymes (slow): enzyme synthesis is induced under conditions of a high protein diet or starvation. [slide states the two conditions but does not explain why they induce the enzymes]
  2. Carbamoyl phosphate synthetase (fast): CPS is allosterically regulated.

The CPS activation cascade:

  • N-acetylglutamate synthase, located in the mitochondria, catalyses acetyl CoA + glutamate to N-acetylglutamate + CoA.
  • N-acetylglutamate allosterically activates CPS, which then drives carbamoyl phosphate formation from CO2 + NH4+ + 2 ATP (releasing 2 ADP + 2 Pi).
  • N-acetylglutamate synthase is upregulated by arginine, and arginine is itself a urea cycle intermediate, so this is a positive feedback loop: a high flux of nitrogen through the cycle raises arginine, which raises N-acetylglutamate, which further activates CPS.
  • The glutamate input can be supplied by glutamine via glutaminase, or used by GDH to give NH4+ feeding the CPS reaction, coupling substrate supply to activation.
  • The majority of the acetyl CoA input comes from beta-oxidation of fatty acids.

Therapeutic implication: because aminotransferase reactions are reversible, they can be exploited in the treatment of urea cycle disorders.

Nitrogen compounds in urine

  • Urea: the major nitrogen excretion product.
  • NH4+: produced in the kidney by deamination of glutamine. It also reduces body acidity, because the process removes protons.
  • Uric acid: the final metabolic product of purine-nucleotide degradation.
  • Creatinine: derived in skeletal muscle by spontaneous cyclisation of creatine and phosphocreatine, releasing Pi and H2O.

Renal ammonia handling in the proximal tubule:

  • Glutamine enters the cell from the lumen via B0AT1, and exits to the interstitium via the SN1 transporter.
  • In the mitochondrion, glutamine goes to glutamate via PDG/glutaminase, releasing NH4+; glutamate goes to alpha-ketoglutarate + NH4+ via GDH.
  • Alpha-ketoglutarate enters the TCA cycle, producing HCO3- and malate. Malate is exported from the mitochondrion to oxaloacetate to PEP to glucose (gluconeogenesis).
  • HCO3- is transported to the interstitium by the Na+-coupled transporter NBCe1A, as (HCO3-)3 with Na+.
  • At the lumen: Na+/NH4+ exchange and Na+/H+ exchange occur via NHE-3, H+ is pumped by an ATP-driven pump, and NH3 effluxes into the lumen.
  • AQP8 allows NH4+/NH3 movement between the mitochondrial matrix and the cytosol.

Net effect: glutamine deamination in the kidney excretes nitrogen as ammonium while generating bicarbonate, so it is simultaneously a nitrogen-disposal and acid-base mechanism.

Degradation of purine nucleotides to uric acid:

  • AMP branch: AMP to adenosine by nucleotidase (losing Pi); adenosine to inosine by adenosine deaminase (losing NH3); IMP can also go directly to inosine by nucleotidase, with NH3 released by a separate route; inosine to hypoxanthine by purine nucleoside phosphorylase (losing ribose-1-phosphate); hypoxanthine to xanthine by xanthine oxidase (using O2, producing H2O2).
  • GMP branch: GMP to guanosine by nucleotidase (losing Pi); guanosine to guanine by purine nucleoside phosphorylase (losing ribose-1-phosphate); guanine to xanthine by guanine deaminase (losing NH3).
  • Both branches converge: xanthine to uric acid by xanthine oxidase (using O2, producing H2O2).
  • The deaminase steps are where the nitrogen is released as ammonia.

Inborn errors of metabolism

General features:

  • Total incidence about 1 in 2000, with ethnic variation for specific disorders.
  • Usually autosomal recessive.
  • The enzyme defect leads to substrate accumulation, sometimes toxic, and product deficiency.
  • Early intervention, where possible, is important.
  • They often lead to neurological deficits.

Newborn metabolic screening (NZ):

  • 28 disorders are screened for.
  • About 50 babies per year in NZ are identified with a metabolic disorder.
  • Diagnostic testing should also be considered if there is a family history.

Phenylketonuria (PKU)

Genetics and biochemistry:

  • Incidence about 1 in 10,000 to 15,000.
  • Deficiency of phenylalanine hydroxylase (PAH), autosomal recessive.
  • Heterogeneous levels of residual activity are associated with different mutations; more than 500 mutations are identified, and 15 common mutations account for 30-50% of cases.
  • PAH is required for the conversion of phenylalanine to tyrosine. With PAH blocked, phenylalanine is instead diverted via transaminase to phenylpyruvic acid and other metabolites.
  • If untreated, blood Phe exceeds 1000 µM, against a normal of about 120 µM.

Clinical observations: microcephaly; delayed development/failure to thrive; cognitive, neurological and behavioural problems; hypomyelination or demyelination; musty odour; pale complexion and hair; skin disorders.

Pathophysiology:

  • The exact mechanism of the brain defects is unclear.
  • Decrease in protein synthesis and in neurotransmitter levels.
  • Excess Phe saturates the blood-brain barrier large neutral amino acid (LNAA) transporters, decreasing LNAAs in the brain.
  • Enzyme inhibition by elevated Phe and its metabolites.

The pale complexion and hair relate to tyrosine being the precursor of melanin, of the catecholamines (tyrosine to dopamine to norepinephrine to epinephrine) and of the thyroid hormones triiodothyronine and thyroxine.

Treatment:

  • Dietary modification: low natural protein, with supplementation using a special dietary formulation low in Phe and high in essential amino acids.
  • Tetrahydrobiopterin (BH4) supplementation: can stabilise the native structure of some mutant forms of PAH, restoring activity. The PAH reaction is L-Phe + BH4 + Fe2+ + O2 to L-Tyr + 4a-OH-BH2.
  • Enzyme therapy: phenylalanine ammonia lyase.
  • Dietary sources must be avoided, including aspartame. Aspartame is L-aspartyl-L-phenylalanine methyl ester, which breaks down to aspartate + phenylalanine + methanol, which is why diet drinks carry the warning “PHENYLKETONURICS: CONTAINS PHENYLALANINE”.

Issues with treatment:

  • The diet is restrictive and expensive.
  • Poor palatability, although improving.
  • Termination of the diet typically occurs at adolescence; there is evidence that diet termination can lead to the development of neurological deficits, and the diet must be reconsidered in the context of pregnancy.
  • It is socially restrictive.

Patient-reported data on living with PKU

Difficulty of management (N = 615): difficult 51.7%, easy 35.5%, neutral 12.9%.

Percentage of blood Phe values during the past year, by age group:

  • Over 18 years: >360 µmol/L 61.5%; 2-6 mg/dL (120-360 µmol/L) 22.7%; not tested 8.7%; not known 5.6%; <2 mg/dL (120 µmol/L) 1.4%.
  • Under 18 years: 2-6 mg/dL (120-360 µmol/L) 67.8%; >360 µmol/L 25.5%; <2 mg/dL (120 µmol/L) 4.0%; not known 2.4%; not tested 0.3%.

Metabolic control is therefore markedly worse in adults than in children.

Most desired lifestyle improvements (percent, count): increase protein intake without increasing PKU symptoms 77.7% (365); eat any foods regardless of protein content 76.0% (357); consume less medical foods 57.7% (271); discontinue use of medical foods 47.2% (222); have better mental health 45.7% (215); improve social relationships 34.5% (162); decrease frequency of blood tests 28.9% (136).

Most desired outcomes from new treatments (percent, count): drop in blood Phe concentrations 87.5% (405); improved attention span and ability to focus 65.7% (304); improved executive function skills 61.6% (285); reduced depression, anxiety and/or mood swings 55.1% (255); improved processing speed 52.1% (241); increase in energy 51.0% (236); improved memory 49.5% (229); lifting of “the fog” 43.0% (199); reduced bone loss 30.0% (139); reduced tremors 19.2% (89); reduced other damage such as muscle weakness and gait disorders 18.8% (87).

Interest in methods of administration of new therapies (no interest / neutral / some-or-strong interest): weekly probiotic 1.7/7.2/91.1; daily oral pills 3.9/7.6/88.5; daily probiotic 2.8/9.5/87.7; monthly injection at home 9.3/4.8/85.9; gene insertion at a medical facility 7.3/7.1/85.7; stem cell infusion over several days at a medical facility 8.1/9.2/82.7; weekly injection at home 14.0/5.7/80.3; monthly injection at a medical facility 16.0/7.0/76.9; daily injection at home 21.7/8.1/70.2; weekly injection at a medical facility 37.7/7.8/54.5; daily injection at a medical facility 50.1/8.0/41.9. The ranking tracks convenience: oral and home-administered routes are strongly preferred over anything requiring frequent visits to a medical facility.

Self-test

  1. Define nitrogen balance, and state the two factors an individual’s nitrogen balance status depends on.
  2. Explain why humans must both replenish and excrete nitrogen every day.
  3. Distinguish positive from negative nitrogen balance in terms of intake, excretion, lean mass and the direction of net flux between body protein and the amino acid pool.
  4. Predict what happens to nitrogen balance when both dietary protein intake and nitrogen excretion rise together, and explain why.
  5. List the inputs to, and the fates of, the body’s amino acid pool.
  6. State the daily whole-body protein turnover and name the organ with the highest fractional replacement rate per day.
  7. Distinguish essential from non-essential amino acids, and give the metabolic origin of alanine, serine, glycine, cysteine and tyrosine.
  8. Explain why cysteine and tyrosine are dependent on dietary essential amino acids despite being classed as non-essential.
  9. Name the amino acid precursors of serotonin, melatonin, histamine and nitric oxide.
  10. List the amino acid precursors of creatine, carnitine and glutathione.
  11. Explain why an amino acid carbon skeleton is classified as glucogenic or ketogenic, and name the amino acid the slides describe as purely ketogenic.
  12. List the five TCA-linked entry points for glucogenic carbon skeletons and one amino acid feeding each.
  13. State three properties of urea that make it a suitable molecule for nitrogen excretion, and say where in the liver it is made.
  14. Describe the reactions catalysed by glutamine synthetase and glutaminase, and say which tissue predominantly performs each in the transport of nitrogen to the liver.
  15. Write the glutamate dehydrogenase reaction and explain its role in feeding nitrogen into the urea cycle.
  16. Explain how a transamination reaction moves nitrogen between molecules, and why its reversibility is therapeutically useful in urea cycle disorders.
  17. Explain the evidence that alanine and glutamine are the main carriers of excess amino groups to the liver.
  18. Describe the muscle-liver-kidney nitrogen shuttle, saying what happens to alanine and to glutamine in each organ.
  19. Distinguish the roles of periportal and perivenous hepatocytes in handling ammonia, and explain why this zonation is effective.
  20. Describe the five steps of the urea cycle in order, naming the enzyme and the compartment for each.
  21. State the two sources of the two nitrogen atoms of urea, and the step of the cycle at which each enters.
  22. Describe how fumarate produced by the urea cycle is used to regenerate aspartate.
  23. Describe the two major points of regulation of the urea cycle, and explain how arginine produces a positive feedback loop on carbamoyl phosphate synthetase.
  24. State the two conditions under which urea cycle enzyme synthesis is induced, and say which of the two points of regulation this represents.
  25. List the nitrogen compounds excreted in urine and state the origin of each.
  26. Explain how renal deamination of glutamine both excretes nitrogen and reduces body acidity.
  27. Describe the degradation of AMP and GMP to uric acid, naming the enzymes at which nitrogen is released.
  28. Explain how ammonia produced in the gut reaches the systemic circulation in portal-systemic shunting, and what the clinical consequence is.
  29. State the general features of inborn errors of metabolism, including typical incidence and inheritance pattern.
  30. Describe the enzyme defect in PKU, the resulting biochemical changes, and the untreated blood Phe concentration compared with normal.
  31. Explain the proposed mechanisms by which elevated phenylalanine damages the brain.
  32. Explain why PKU patients have a pale complexion and hair, using the products derived from tyrosine.
  33. List the three treatment strategies for PKU, and give one practical or social issue with the dietary approach.
  34. Explain why aspartame-sweetened products carry a warning for people with PKU.
  35. Integrative: Mr Griffin has active Crohn’s disease with multiple bowel resections, fistulas and ongoing inflammation, and was in negative nitrogen balance before TPN. Explain his negative balance, why TPN improved it, why added glutamine helped further, and why he might be at increased risk of hyperglycaemia.

Answers

Graph View

Backlinks