The cyclase of Aplysia californica catalyzed an alternative mode of cyclization of nicotinamide guanine dinucleotide (NGD) and nicotinamide hypoxanthine dinucleotide (NHD) to form cyclic-GDP-ribose (cGDPR) and cyclic-HDP-ribose (cHDPR) respectively. In these cyclic nucleotides, the newly formed glycosyl bonds are attached onto the N-7 nitrogen of the purine rings instead of the N-1 nitrogen as in cyclic ADP-ribose (cADPR).