Probing Aplysia californica Adenosine 5‘-Diphosphate Ribosyl Cyclase for Substrate Binding Requirements:  Design of Potent Inhibitors

Readily synthesized nicotinamide adenine dinucleotide (NAD+) analogues have been used to investigate aspects of the cyclization of NAD+ to cyclic adenosine 5‘-O-diphosphate ribose (cADPR) catalyzed by the enzyme adenosine 5‘-O-diphosphate (ADP) ribosyl cyclase and to produce the first potent inhibitors of this enzyme. In all cases, inhibition of Aplysia californica cyclase by various substrate analogues was found to be competitive while inhibition by nicotinamide exhibited mixed-behavior characteristics. Nicotinamide hypoxanthine dinucleotide (NHD+), nicotinamide guanine dinucleotide (NGD+), C1‘-m-benzamide adenine dinucleotide (Bp2A), and C1‘-m-benzamide nicotinamide dinucleotide (Bp2N) were found to be nanomolar potency inhibitors with inhibition constants of 70, 143, 189, and 201 nM, respectively. However, NHD+ and NGD+ are also known substrates and are slowly converted to cyclic products, thus preventing their further use as inhibitors. The symmetrical bis-nucleotides, bis-adenine dinucleotide (Ap2A), bis-hypoxanthine dinucleotide (Hp2H), and bis-nicotinamide dinucleotide (Np2N), exhibited micromolar competitive inhibition, with Ap2A displaying the greatest affinity for the enzyme. 2‘,3‘-Di-O-acetyl nicotinamide adenine dinucleotide (AcONAD+) was not a substrate for the A. californica cyclase but also displayed some inhibition at a micromolar level. Finally, inhibition of the cyclase by adenosine 5‘-O-diphosphate ribose (ADPR) and inosine 5‘-O-diphosphate ribose (IDPR) was observed at millimolar concentration. The nicotinamide aromatic ring appears to be the optimal motif required for enzymatic recognition, while modifications of the 2‘- and 3‘-hydroxyls of the nicotinamide ribose seem to hamper binding to the enzyme. Stabilizing enzyme/inhibitor interactions and the inability of the enzyme to release unprocessed material are both considered to explain nanomolar inhibition. Recognition of inhibitors by other ADP ribosyl cyclases has also been investigated, and this study now provides the first potent nonhydrolyzable sea urchin ADP ribosyl cyclase and cADPR hydrolase inhibitor Bp2A, with inhibition observed at the micromolar and nanomolar level, respectively. The benzamide derivatives did not inhibit CD38 cyclase or hydrolase activity when NGD+ was used as substrate. These results emphasize the difference between CD38 and other enzymes in which the cADPR cyclase activity predominates.