Fluorescent analogs of NAADP with calcium mobilizing activity

Abstract

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes Ca 2+ through a mechanism totally independent of cyclic ADP-ribose or inositol trisphosphate. Fluorescent analogs of NAADP were synthesized in this study to facilitate further characterization of this novel Ca 2+ release mechanism. The base-exchange reaction catalyzed by ADP-ribosyl cyclase was utilized to convert nicotinamide 1, N 6-ethenoadenine dinucleotide phosphate to a fluorescent product, nicotinic acid 1, N 6-ethenoadenine dinucleotide phosphate (etheno-NAADP). The excitation spectrum of the product showed two maxima at 275 nm and 300 nm and an emission maximum at 410 nm. An aza derivative of etheno-NAADP was also synthesized by sequential treatments with NaOH and nitrite. The product, nicotinic acid 1, N 6-etheno-2-aza-adenine dinucleotide phosphate (etheno-aza-NAADP) had excitation maxima at 280 nm and 360 nm and an emission maximum at 470 nm. The fluorescence of both analogs was sensitive to polarity and exhibited a 3–4-fold enhancement going from an aqueous buffer to an organic solvent. Proton-NMR measurements confirmed the presence of the etheno ring in both analogs. In the aza derivative the proton at the 2-position of the adenine ring was absent, consistent with the conversion of the 2-carbon to a nitrogen. Both analogs could activate Ca 2+ release from sea urchin egg homogenates and the half-maximal concentrations for etheno-aza-NAADP and etheno-NAADP were at about 2.5 μM and 5 μM, respectively. At sub-threshold concentrations, both analogs could also function as antagonists, inactivating the NAADP-sensitive Ca 2+ release with a half-maximal concentration of 60–80 nM. Microinjection of etheno-aza-NAADP into live eggs activated Ca 2+ increase and triggered a cortical exocytotic reaction confirming its effectiveness in vivo. These fluorescent analogs are potentially useful for visualizing the novel Ca 2+ stores that are sensitive to NAADP in live cells.