The inhibition in vitro of l-asparagine synthetase (l-glutamine hydrolyzing, EC 220.127.116.11) from leukemia 5178Y rendered resistant to l-asparaginase (L5178Y/AR), and from mouse pancreas by the ketoamino acids DON (l-DON; 6-diazo-5-oxo-l-norleucine), CONV (l-CONV; 2-amino-5-chlorolevulinic acid; 5-chloro-4-oxo-l-norvaline) and DONV (l-DONV; 5-diazo-4-oxo-l-norvaline) was investigated using both l-glutamine and ammonium chloride as substrates. At a concentration of 1 mM, DON and CONV almost completely inhibited the utilization in vitro of l-glutamine by l-asparagine synthetase of L5178Y/AR and of mouse pancreas, whereas DONV inhibited both enzymes only by 50 per cent. DON, CONV and DONV did not affect the utilization in vitro of ammonium chloride by l-asparagine synthetase of L5178Y/AR, while DON and CONV modestly inhibited the utilization of ammonium chloride by the pancreatic enzyme. The inhibition produce by DONV was fully and rapidly reversed by dialysis, whereas that produced by DON and CONV was essentially irreversible. The utilization in vitro of l-glutamine by eight fetal rat liver amidotransferases was strongly inhibited by DON and CONV, while DONV exerted modest inhibition on only two of these enzymes. In a survey of other enzymes which use l-asparagine and l-glutamine as substrates, DONV was shown to be the best inhibitor of l-asparagine-utilizing enzymes, DON was the best inhibitor of enzymes utilizing l-glutamine, and CONV affected both groups of enzymes to a variable degree. DON irreversibly inhibited l-asparaginase from Erwinia carotovora (EC 18.104.22.168). This enzyme also was found to catalyze the decomposition of DON. From these findings and also from kinetic studies, it is clear that all three ketoamino acids are capable of behaving as antagonists of l-glutamine in vitro. DON is the most universally active antagonist of this amino acid, followed by CONV, and last by DONV. Similarly, all three agents can function as l-asparagine antagonists under appropriate conditions.
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