Mechanism-based inhibitors of CD38: A mammalian cyclic ADP-ribose synthetase

The soluble domain of human CD38 catalyzes the conversion of NAD⁺ to cyclic ADP-ribose and to ADP-ribose via a common covalent intermediate [Sauve, A. A., Deng, H. T., Angelletti, R. H., and Schramm, V. L. (2000) J. Am. Chem. Soc. 122, 7855-7859]. Here we establish that mechanism-based inhibitors can be produced by chemical stabilization of this intermediate. The compounds nicotinamide 2′-deoxyriboside (1), 5-methylnicotinamide 2′-deoxyriboside (2), and pyridyl 2′-deoxyriboside (3) were synthesized and evaluated as inhibitors for human CD38. The nicotinamide derivatives 1 and 2 were inhibitors of the enzyme as determined by competitive behavior in CD38-catalyzed conversion of nicotinamide guanine dinucleotide (NGD⁺) to cyclic GDP-ribose. The K_i values for competitive inhibition were 1.2 and 4.0 μM for 1 and 2, respectively. Slow-onset characteristics of reaction progress curves indicated a second higher affinity state of these two inhibitors. Inhibitor off-rates were slow with rate constants k_off of 1.5 × 10^-5 s^-1 for 1 and 2.5 × 10^-5 s^-1 for 2. Apparent dissociation constants K_i(total) for 1 and 2 were calculated to be 4.5 and 12.5 nM, respectively. The similar values for k_off are consistent with the hydrolysis of common enzymatic intermediates formed by the reaction of 1 and 2 with the enzyme. Both form covalently attached deoxyribose groups to the catalytic site nucleophile. Chemical evidence for this intermediate is the ability of nicotinamide to rescue enzyme activity after inactivation by either 1 or 2. A covalent intermediate is also indicated by the ability of CD38 to catalyze base exchange, as observed by conversion of 2 to 1 in the presence of nicotinamide. The deoxynucleosides 1 and 2 demonstrate that the chemical determinants for mechanism-based inhibition of CD38 can be satisfied by nucleosides that lack the 5′-phosphate, the adenylate group, and the 2′-hydroxyl moiety. In addition, these compounds reveal the mechanism of CD38 catalysis to proceed by the formation of a covalent intermediate during normal catalytic turnover with faster substrates. The covalent 2′-deoxynucleoside inactivators of CD38 are powerful inhibitors by acting as good substrates for formation of the covalent intermediate but are poor leaving groups from the intermediate complex because hydrolytic assistance of the 2′-hydroxyl group is lacking. The removal of the adenylate nucleophile required for the cyclization reaction provides slow hydrolysis as the only exit from the covalent complex.