ADP-ribonolactone: A potential activated intermediate analogue of NAD-glycohydrolase

Aldono-l,S-lactones are efficient competitive inhibitors with high specificity for glycosidases that catalyze the hydrolysis of glycosides derived from the same aldoses [ 11. The inhibition has been attributed to a conformational (half-chair) and electronic (sp2 hybridization and electron delocalization) similarity between the lactone and the hypothetical glycosyl-ion which is involved as an intermediate in substrate transformation [2]. A rationale was provided by application of absolute reaction rates theory to enzymatic catalysis. It predicts that an enzyme develops an enhanced affinity for the transition state of a catalyzed reaction, and therefore for ‘transition state analogues’, comparatively to its substrates [3]. A classical example for glycosidases is that of lysozyme where the glycosyl group at which cleavage occurs must be distorted, according to X-ray analysis, into a half-chair conformation in order to fit into the active site. This substrate distortion towards the structure of the transition state is believed to be a major factor responsible for the mode of action of that enzyme [4]. A 1,5-lactone derived of chitotetraose, an analogue for the proposed transition-state, was found to a potent ligand for lysozyme [ 51. NAD-glycohydrolases (EC 3.2.2.6) catalyze the hydrolysis of NAD at the nicotinamide-ribose linkage. The kinetic mechanism of calf spleen NAD glycohydrolase was found to be consistent with an Ordered Uni Bi, the formation of an ADP-ribosyl intermediary complex (E-ADP-rib) being rate determining [6]. The reactivity of E-ADP-rib versus nucleophiles was in favor of an oxocarbonium ion intermediate in the NAD transformation [6,7] . In