CRYSTALLOGRAPHIC STUDIES OF 2 ALCOHOL DEHYDROGENASE-BOUND ANALOGS OF THIAZOLE-4-CARBOXAMIDE ADENINE-DINUCLEOTIDE (TAD), THE ACTIVE ANABOLITE OF THE ANTITUMOR AGENT TIAZOFURIN

Thiazole-4-carboxamide adenine dinucleotide (TAD) is the active anabol ite of the antitumor drug tiazofurin. Beta-methylene TAD (beta-TAD) is a phosphodiesterase-resistant analogue of TAD, active in tiazofurin-r esistant cells. Beta-methylene SAD (beta-SAD) is the active selenium d erivative of beta-TAD. Both agents are analogues of the cofactor NAD a nd are capable of acting as general dehydrogenase inhibitors. Crystal structures of beta-TAD and beta-SAD bound to horse liver alcohol dehyd rogenase (LADH) are presented at 2.9 and 2.7 angstrom, respectively. B oth complexes crystallize in the orthorhombic space group C222(1) and are isomorphous to apo-LADH. Complexes containing beta-TAD and beta-SA D were refined to crystallographic R values of 15% and 16%, respective ly, for reflections between 8 angstrom and the minimum d spacing. Conf ormations of both inhibitors are similar. Beta-TAD and beta-SAD bind t o the ''open'' form of LADH in the normal cofactor-binding cleft betwe en the coenyzme and catalytic domains of each monomer. Binding at the adenosine end of each inhibitor resembles that of NAD. However, the po sitions of the thiazole and selenazole heterocycles are displaced away from the catalytic Zn cation by approximately 4 angstrom. Close intra molecular S-O and Se-O contacts observed in the parent nucleoside anal ogues are maintained in both LADH-bound beta-TAD and beta-SAD, respect ively. These conformational constraints may influence the binding spec ificity of the inhibitors.