Catalytic properties of the retinal rod outer segment disk ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR) is a second messenger modulating intracellular calcium levels. We have previously described a cADPR-dependent calcium signaling pathway in bovine rod outer segments (ROS), where calcium ions play a pivotal role. ROS ADP-ribosyl cyclase (ADPR-cyclase) was localized in the membrane fraction. In the present work, we examined the properties of the disk ADPR-cyclase through the production of cyclic GDP-ribose from the NAD+ analogue NGD+. The enzyme displayed an estimated Km for NGD+ of 12.5 6 0.3 lM, a Vmax of 26.50 6 0.70 pmol cyclic GDP-ribose synthesized/min/mg, and optimal pH of 6.5. The effect of divalent cations (Zn2+, Cu2+, and Ca2+) was also tested. Micromolar Zn2+ and Cu2+ inhibited the disk ADPR-cyclase activity (half maximal inhibitory concentration, IC50 5 1.1 and 3.6 lM, respectively). By contrast, Ca2+ ions had no effect. Interestingly, the properties of the intracellular membrane–associated ROS disk ADPR-cyclase are more similar to those of the ADPR-cyclase found in CD38-deficient mouse brain, than to those of CD38 or CD157. The novel intracellular mammalian ADPR-cyclase would elicit Ca2+ release from the disks at various rates in response to change in free Ca2+ concentrations, caused by light versus dark adaptation, in fact there was no difference in disk ADPR-cyclase activity in light or dark conditions. Data suggest that disk ADPR-cyclase may be a potential target of retinal toxicity of Zn2+ and may shed light to the role of Cu2+ and Zn2+ deficiency in retina.