Some Enzymes of the Urea Cycle

Arginine or citrulline in concentration in excess of the optimal can inhibit both argininosuccinate synthetase or argininosuccinate lyase in tissue cultures (S6). Whether this is applicable to the conditions of synthesis of the urea in liver is uncertain. a-Methylaspartic acid also specifically inhibits argininosuccinate synthetase (B7, C4, SIO). 

Lysine as well as ornithine and citrulline can inhibit arginase (C6, H5, S2). In the same way, citrulline can inhibit ornithine transcarbamylase (B13), which surprisingly is also inhibited by excess ornithine (S12). 

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