Toxic action of ammonia
Ammonia exerts multifaceted toxic effects, most significantly in nervous tissue. The main mechanism is associated with enhanced synthesis of glutamate from α-ketoglutarate and subsequently glutamine. Externally this looks favorable (increase in glutamate, the precursor of GABA), but the consequences are predominantly negative.
- Reduction of the α-ketoglutarate pool → inhibition of the tricarboxylic acid cycle (TCA cycle) and development of a hypoenergetic state in the brain.
- Decreased rate of transamination reactions, since α-ketoglutarate is the main acceptor of amino groups.
- Shift of the «glutamate → glutamine» equilibrium towards glutamine → glutamate deficiency → decreased synthesis of GABA (inhibitory neurotransmitter) → convulsive readiness and impaired inhibitory impulses.
- Accumulation of glutamine increases osmotic pressure in neurons and astrocytes, contributing to brain edema.
- Protonation of NH₃ with the formation of the ammonium ion shifts the pH in the alkaline direction → alkalosis.
Pathways of utilization of amino acid carbon skeletons
Based on the fate of their carbon skeleton, amino acids are divided into three groups:
- Glucogenic — yield pyruvate or TCA cycle metabolites and are used for gluconeogenesis: glycine, alanine, serine, cysteine, aspartate, asparagine, glutamate, etc.
- Ketogenic — are converted into acetyl-CoA or acetoacetate: leucine and lysine.
- Mixed — yield both glucose and ketone bodies: isoleucine, phenylalanine, tyrosine, tryptophan.
The entry of amino acids into the TCA cycle via α-ketoglutarate (from glutamate), succinyl-CoA (from valine, methionine), fumarate, and oxaloacetate constitutes the so-called anaplerotic reactions, which maintain the pool of cycle intermediate metabolites.
Serine and glycine metabolism. Folic acid
Serine is a non-essential amino acid, synthesized from 3-phosphoglycerate (an intermediate of glycolysis) via dehydrogenation, transamination, and hydrolysis. It is catabolized to pyruvate, then further to acetyl-CoA or glucose.
Glycine is formed from serine by the action of serine hydroxymethyltransferase; the coenzyme is tetrahydrofolate (H₄-folate), which accepts the β-carbon atom of serine to form N⁵,N¹⁰-methylene-H₄-folate.
H₄-folate derivatives transfer single-carbon units:
- methylene-H₄-folate — synthesis of dTMP and regeneration of methionine;
- methenyl- and formyl-H₄-folate — synthesis of purine nucleotides.
Catabolism of glycine (glycine synthase) yields CO₂, NH₃, and methylene-H₄-folate.
H₄-folate synthesis and sulfonamides
From folic acid in the liver, H₄-folate is formed by two successive NADPH-dependent reductions: first, folate reductase yields H₂-folate, then dihydrofolate reductase yields H₄-folate. The latter enzyme is inhibited by the antitumor drug methotrexate.
Folic acid hypovitaminosis occurs with insufficient vegetable intake, impaired absorption (absence of Castle's intrinsic factor, hepatitis, cirrhosis). The deficiency reduces nucleotide synthesis, which is critical for rapidly dividing cells (erythropoiesis) and leads to megaloblastic anemia.
Sulfonamides are structural analogues of para-aminobenzoic acid (PABA), a precursor of folic acid in bacteria. They act as competitive inhibitors of bacterial folate synthesis enzymes. Humans obtain folate from diet, so the drugs exhibit selective bacteriostatic action.
Methionine and S-adenosylmethionine metabolism
Methionine is an essential amino acid, initiates translation (AUG codon), and serves as the main donor of methyl groups. The active form is S-adenosylmethionine (SAM), formed from methionine and ATP by the action of methionine adenosyltransferase. After abstraction of the methyl group, it is converted into S-adenosylhomocysteine (SAH), and then into homocysteine.
Regeneration of methionine from homocysteine requires methyl-H₄-folate and vitamin B₁₂, which links methionine metabolism with the metabolism of serine, glycine, and folate. An alternative pathway is the conversion of homocysteine to cysteine involving pyridoxal phosphate (B₆).
Genetic enzyme defects or folate and B₆ hypovitaminosis lead to homocystinuria: accumulation of homocysteine in blood and tissues, ectopia lentis, osteoporosis, thrombosis.
Biological functions of SAM
SAM participates in the synthesis of numerous methylated compounds:
- Phosphatidylcholine — three successive methylations of phosphatidylethanolamine (consumption of three SAM molecules).
- Carnitine — transporter of acyl groups into mitochondria; formed from lysine with triple methylation.
- Creatine — synthesized from arginine, glycine, and methionine; phosphorylated in muscles to creatine phosphate, which serves as a rapid reserve for ATP regeneration.
- Synthesis of adrenaline, methylation of DNA and RNA bases.
- Detoxification of toxic and medicinal substances via their methylation.
Key points
- The toxicity of ammonia is realized through depletion of α-ketoglutarate, inhibition of the TCA cycle, GABA deficiency, and osmotic brain edema.
- Amino acids are divided into glucogenic, ketogenic (Leu, Lys), and mixed; their metabolites replenish the TCA cycle.
- H₄-folate is a coenzyme for the transfer of single-carbon units for nucleotide synthesis and methionine regeneration.
- Methotrexate inhibits human dihydrofolate reductase, while sulfonamides inhibit folate synthesis in bacteria.
- Folic acid deficiency causes megaloblastic anemia due to impaired nucleotide synthesis.
- SAM is a universal methyl group donor; it is converted into SAH and homocysteine.
- Remethylation of homocysteine requires B₁₂ and methyl-H₄-folate; synthesis of cysteine requires B₆.
- Methionine is essential for the synthesis of phosphatidylcholine, carnitine, creatine, adrenaline, and detoxification reactions.