Scintillation counting of the ninhydrin-amino acid-C14 reaction products from an automatic amino acid analyzer

Metabolism of arginine in lactating rat mammary gland

Significant activities of the four enzymes needed to convert arginine into proline and glutamate (arginase, ornithine aminotransferase, pyrroline-5-carboxylate reductase and pyrroline-5-carboxylate dehydrogenase) develop co-ordinately in lactating rat mammary glands in proportion to the increased production of milk. No enzymes were detected to carry out the reactions of proline oxidation or reduction of glutamate to pyrroline-5-carboxylate. Minces of the gland converted ornithine into proline and into glutamate plus glutamine. These conversions increased during the cycle of lactation in proportion to the increased milk production and to the content of the necessary enzymes. The minced gland did not convert labelled ornithine into citrulline, confirming the absence from the gland of a functioning urea cycle, and did not convert labelled proline or glutamate into ornithine. A metabolic flow of labelled arginine to proline and glutamate in mammary gland was confirmed in intact animals with experiments during which the specific radioactivity of proline in plasma remained below that of the proline being formed from labelled arginine within the gland. It was concluded that arginase in this tissue had a metabolic role in the biosynthesis of extra proline and glutamate needed for synthesis of milk proteins.

Amino acid biosynthesis in the spirochete Leptospira: evidence for a novel pathway of isoleucine biosynthesis

Radioactive carbon dioxide was incubated with growing cells of Leptospira interrogans serotypes semaranga and tarassovi, and the specific activities and distribution of the label within the cellular amino acids were determined. The origins of the carbon skeletons of all the acid-stable amino acids except isoleucine were found to be consistent with known biosynthetic pathways for these amino acids. Experiments using radioactive carbon dioxide and other tracers indicated that most of the isoleucine was synthesized by a pathway not involving threonine. The origin of the carbon skeleton of isoleucine consisted of two residues of pyruvate (carbons 2 and 3) and acetate of acetyl-coenzyme A by this pathway. Isotope competition studies indicated that the pathway was regulated by isoleucine. The results are discussed in relation to two proposed pathways of isoleucine biosynthesis involving citramalate as an intermediate.