Characterization of NAADP(+) binding in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP(+)) is a pyridine nucl eotide which has been shown to release Ca2+ from intracellular membranes in echinoderms, Ascidiae, mammals, and plants. NAADP releases Ca2+ via a mech anism independent of ryanodine and inositol 1,4,5-trisphosphate (IP3) recep tors and the NAADP(+) receptor is likely to be located on a separate organe lle. We have investigated the binding characteristics of NAADP(+) to its re ceptor in sea urchin egg homogenates. NAADP(+) binds to a saturable membran e-bound site with high affinity (K-d = 193 +/- 35.7 pM). NAADP(+) associate s to its receptor with a t(1/2) of approximately 7 min while dissociation d oes not occur during the time course of the experiment. Furthermore, NAD(+) , NAAD(+), ADP, or ATP cannot displace NAADP(+) binding. The structurally r elated molecules NADP(+) and NADPH displayed a markedly lower affinity for the binding site with K-d's 500- and 25,000-fold higher than NAADP(+), resp ectively. This discrepancy between oxidized and reduced forms of NADP(+) mi ght suggest that NAADP(+) signaling is itself regulated by the redox state of the cell. (C) 2000 Academic Press.