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Effects of analogues of aspartic acid on enzymes of urea synthesis

1968; Elsevier BV; Volume: 127; Linguagem: Inglês

10.1016/0003-9861(68)90279-8

1096-0384

Olga Rochovansky, S. Ratner,

Enzyme function and inhibition

Abstract Analogues of aspartic acid have been studied as inhibitors of urea synthesis starting from citrulline and aspartate in homogenates of rat liver and their effects compared with those on the isolated enzymes. The analogue α-methyl- d , l -aspartate behaved as a weak, competitive inhibitor, consistent with expectation based on the Ki for argininosuccinate synthetase. The site of inhibition of urea synthesis is thus confined to the synthetase site. The compound threo-β-methyl- l -aspartate is a substrate for the synthetase and gives rise to the methyl analogue of argininosuccinate, arginino-β-methylsuccinate. The latter compound was prepared enzymatically; it acts as inhibitor for argininosuccinase, in competition with argininosuccinate. The weak inhibition of urea synthesis observed in the presence of β-methyl- l -aspartate was consistent with the Km of this analogue for the isolated synthetase. The site of inhibition is consequently confined mainly to the synthetase site and arises from competition between the two substrates. Little inhibition occurs at the argininosuccinase site due to the low level of arginino-β-methylsuccinate formed and to the appreciable steady-state level of argininosuccinate found to be present during urea synthesis. Administration of either α-methyl- or β-methylaspartate to rats by intraperitoneal or intracardiac injection produced no effects on urea synthesis. The effects of the threo and erythro forms of β-hydroxy- d , l -aspartate and β-hydroxy-β-methyl- d , l -aspartate on isolated argininosuccinate synthetase were examined. Only threo-β-hydroxy- d , l -aspartate was active as substrate. The remaining three compounds acted as inhibitors. The relationship of configuration to enzyme affinity is discussed.