Amino Acid Metabolism Amino Acid Synthesis

Microorganisms in the digestive tract can synthesize all amino acids in the presence of ammonia, sulfur, and carbohydrates.[2] All animals can synthesize tyrosine as well as the following amino acids and their carbon skeletons: alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, and serine. The ability to synthesize citrulline and its carbon skeleton varies among species, but arginine can be made from citrulline in all animal cells.

Because of its large mass (representing 45% of adult body weight), skeletal muscle accounts for the majority of glutamine and alanine synthesis from branched-chain amino acids (BCAA) in animals. These synthetic pathways also occur in extrahepatic tissues, including the brain, adipose tissue, intestine, kidney, lung, placenta, and lactating mammary gland. The liver and kidney are the major sites for the synthesis of tyrosine from phenylala-nine by phenylalanine hydroxylase, whereas hepatic transsulfuration is primarily responsible for cysteine synthesis from methionine. There is no conversion of tyrosine into phenylalanine or cysteine into methionine. In contrast, there is reversible interconversion of serine into glycine by hydroxymethyltransferase in tissues, including the liver, kidney, lactating mammary tissue, placenta, and intestine. Proline can be synthesized from arginine in animal cells containing mitochondria, and from gluta-mine and glutamate in most mammals (e.g., pigs and ruminants).[3]

Utilization of precursors for the synthesis of L-amino acids is of practical importance in animal production. Most D-amino acids, except for D-lysine, D-threonine, D-cystine, D-arginine and D-histidine, can be converted into L-amino acids in animals via widespread D-amino acid oxidase and transamination.1-4-1 The efficiency of D-amino acid utilization, on a molar basis of the L-isomer, may be 20 to 100%, depending on species and substrates. Most of the a-ketoacids can be transaminated to form L-amino acids in animals.

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