Comparative Study of Amino Acid Catabolism in Mammalian Tissues: Metabolic Flux and Enzymatic Control

Background: Amino acid catabolism plays a critical role in energy production, nitrogen balance, and biosynthesis of key metabolites across mammalian tissues. Each tissue exhibits unique enzymatic profiles and metabolic fluxes reflecting its physiological roles, substrate preferences, and hormonal regulation. Understanding the comparative landscape of amino acid degradation pathways across organ systems provides essential insights into systemic metabolic coordination, tissue-specific vulnerabilities, and the pathophysiology of inherited and acquired metabolic disorders. Methods and Results: This review integrates data from isotope tracing, transcriptomics, proteomics, and enzyme activity assays to compare amino acid catabolic flux across major mammalian tissues including liver, kidney, muscle, brain, and intestine. The liver serves as the central hub for transamination, urea cycle function, and gluconeogenic conversion of glucogenic amino acids such as alanine and glutamine. In contrast, skeletal muscle preferentially catabolizes branched-chain amino acids (BCAAs: leucine, isoleucine, valine) via mitochondrial branched-chain α-keto acid dehydrogenase complexes, while the kidney contributes significantly to glutaminolysis and ammonia excretion. The brain displays tight regulation of neurotransmitter-associated amino acids such as glutamate and tryptophan, with high expression of aminotransferases and decarboxylases. Tissue-specific enzyme expression patterns, cofactor availability, redox status, and hormonal cues (e.g., insulin, glucocorticoids) collectively shape catabolic flux. Disorders such as maple syrup urine disease, phenylketonuria, and hyperammonemia illustrate the pathological consequences of enzymatic deficiencies in amino acid metabolism. Recent advances in metabolic modeling and multi-omics integration are enhancing our ability to quantify dynamic flux and predict tissue-specific responses to dietary or pharmacological interventions. Conclusion: Comparative analysis of amino acid catabolism across mammalian tissues reveals finely tuned enzymatic networks adapted to organ-specific metabolic demands.