Microorganisms in the digestive tract degrade all amino acids, with ammonia, fatty acids (including branched-chain fatty acids, acetate, propionate and butyrate), H2S, and CO2 being major products. In animals, amino acids are catabolized by cell- and tissue-specific pathways. The liver is the principal organ for the catabolism of all amino acids except for BCAA and glutamine. There is growing recognition that the mammalian small intestine extensively degrades essential and nonessential amino acids, such that circulating glutamate, aspartate, and glutamine arise almost entirely from endogenous synthesis.
Although each amino acid has its own unique catabolic pathway(s), the catabolism of all amino acids exhibits a number of common characteristics (Table 1). Their important products include glucose, ketone bodies, fatty acids, urea, uric acid, and other nitrogenous substances (Table 2). Complete oxidation of amino acids occurs only if their carbon skeletons are ultimately converted to acetyl-CoA, which is oxidized via the Krebs cycle. On a molar basis, oxidation of amino acids is less efficient for ATP production compared with fat and glucose. Gluta-mine, however, is a major fuel for rapidly dividing cells, including enterocytes, immunologically activated lymphocytes, and tumors.
Ammonia is an essential substrate in intermediary metabolism, but at high concentrations it is toxic to animal cells (particularly in the brain). Thus, plasma levels of ammonia (primarily NH+) must be precisely regulated. Syntheses of urea (via hepatic and intestinal urea cycles) and uric acid (via hepatic purine metabolism) represent the major pathways for ammonia detoxification in mammals and birds, respectively. Hepatic ureagenesis is subject to both short- and long-term regulation: 1) availabilities of substrates and N-acetylglutamate, and 2) adaptive changes in the amounts of urea cycle enzymes. Glutamine synthetase is a major regulatory enzyme for uric acid synthesis in uricotelic species.
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