Unique kinetics of nicotinic acid-adenine dinucleotide phosphate (NAADP) binding enhance the sensitivity of NAADP receptors for their ligand

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is a novel and potent Ca2+-mobilizing agent in sea urchin eggs and other cell types. Little is k nown, however, concerning the properties of the putative intracellular NAAD P receptor. In the present study we have characterized NAADP binding sites in sea urchin egg homogenates. [P-32]NAADP bound to a single class of high- affinity sites that were reversibly inhibited by NaCl but insensitive to pH and Ca2+. Binding of [P-32]NAADP was lost in preparations that did not mob ilize Ca2+ in response to NAADP, indicating that [P-32]NAADP probably binds to a receptor mediating Ca2+ mobilization. Addition of excess unlabelled N AADP, at various times after initiation of [P-32]NAADP binding, did not res ult in displacement of bound [P-32]NAADP. These data show that NAADP become s irreversibly bound to its receptor immediately upon association. Accordin gly, incubation of homogenates with low concentrations of NAADP resulted in maximal labelling of NAADP binding sites. This unique property renders NAA DP receptors exquisitely sensitive to their ligand, thereby allowing detect ion of minute changes in NAADP levels.