Role of the nicotinic acid group in NAADP receptor selectivity.

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to be an intracellular Ca2+-releasing messenger in a wide variety of systems to date. Its actions are both potent and highly specific despite differing structurally from the endogenous cellular co-factor and its precursor, NADP, only in the substitution of a hydroxyl for the amine group at the 3' position of the pyridine ring. This substitution allows NAADP to bind to a membrane-localized binding site in sea urchin egg homogenates with an IC50 at least 1000-fold greater than that of NADP as measured by competition radioligand binding assays. This suggests that the NAADP receptor protein must include certain features in the NAADP binding site that regulate this specificity. In order to investigate this interaction, we synthesised a series of NAADP analogues differing from NAADP at the 3' position of the pyridine ring that included both simple carboxylic acid analogues as well as a series of chemical isosters. We then investigated both their affinity for the NAADP binding site in sea urchin egg homogenates and their ability to activate the NAADP sensitive Ca2+ channel. We hereby show that a negative charge at the 3' position is an important determinant of affinity but the protein displays a large tolerance for the size of the group. Furthermore, the protein does not easily accommodate multiple charged groups or large uncharged groups.