The mechanism of arginine synthesis from citrulline in kidney.

The synthesis of arginine from citrulline by the mammalian kidney was first demonstrated by Borsook and Dubnoff (1) using tissue slices. The successful use of kidney homogenates was later. reported by Cohen and Hayano (2). As with arginine formation in liver, the studies with respiring tissue did not clearly reveal t’he nature of the reactions involved. It was of interest to see whether the enzymatic mechanism occurring in liver (3) applies also to bissues and organisms which lack the other enzymes of the ornithine cycle. In the present study, arginine formation has been investigated in respiring kidney homogenates, in crude exbracts of acetone-dried tissue, and in fractionated preparations. Nature of Reaction Mechanism-Arginine is formed in the liver by an ATP-dependent’ condensation of citrulline with aspartic acid to form an intermediary condensation product, which is then split by a separate enzyme. The cleavage of the intermediate was considered to be a hydrolytic reaction, resulting in the production of malic acid simultaneously with arginine (3, 4). It has since been found that the cleavage results in the formation of fumaric acid and is therefore non-hydrolytic. The presence of fumarase in extracts of acetone-dried tissue and in crude preparations of the splitt.ing enzyme accounts for the appearance of a mixture of fumarate and malate, the latter predominating in accord with fumarase equilibrium (5). The previous estimations of malic acid (4) by means of malate-adapted Lactobacillus arabinosus included fumaric acid, since these organisms contain fumarase (6, 7). That the over-all conversion of citrulline to arginine occurs according to this stepwise mechanism in kidney extracts is shown by the data in Table I. Experiment 1 demonstrates the formation of malic and fumaric acids in amounts equivalent to arginine production. It may be seen from