Latent inhibitors. Part 4. Irreversible inhibition of dihydro-orotate dehydrogenase by hydantoins derived from amino acids

Abstract

Hydantoins and ureas derived from α-amino acids are shown to interact with dihydro-orotate dehydrogenase from Clostridium(Zymobacterium)oroticum, chiefly as weak competitive inhibitors but that the hydantoin derived from phenylalanine, 5-benzyl-3-(1-carboxy-2-phenylethyl)hydantoin, is a time-dependent irreversible inhibitor of the enzyme. Inhibition experiments with derivatives of this hydantoin and also its intrinsic chemical reactivity lead to a hypothetical mechanism for the inhibition reaction involving benzylic oxidation of the hydantoin followed by Michael addition of an enzymic nucleophile. The relationship between this reaction and the normal substrate oxidation reaction is discussed in the light of a comparison of substrate and inhibitor structure by molecular graphics.