Dehydrogenase binding by tiazofurin anabolites

Thiazole-4-carboxamide adenine dinucleotide (TAD) is the active anabolite of the new antitumor agent tiazofurin (NSC 286193). TAD is an analogue of NAD in which the nicotinamide ring has been replaced by a thiazole-4-carboxamide heterocycle. TAD putatively acts by inhibition of inosine monophosphate dehydrogenase (IMPd). In this study it is shown that TAD is a competitive inhibitor, with respect to NAD, of mammalian glutamate, alcohol, lactate, and malate dehydrogenases. TAD binds to these enzymes with 1-2 orders of magnitude less affinity than it binds to IMPd. Computer modeling studies suggest that dehydrogenase binding by TAD occurs at the regular cofactor site, the thiazole-4-carboxamide group mimicking the steric and hydrogen-bonding properties of the nicotinamide ring. Noncompetitive kinetics of TAD inhibition of the target enzyme IMPd are potentially due to a reverse order of addition of substrate and cofactor from that observed in the dehydrogenases studied here. The weaker binding of TAD to these dehydrogenases may be due to their inability to preserve a close sulfur-oxygen contact in the bound inhibitor.