Sperm express a Ca 2+}-regulated NAADP synthase
Résumé
NAADP (nicotinic acid adenine dinucleotide phosphate), the most potent Ca 2+}-mobilizing second messenger, is active in a wide range of organisms and cell types. Until now, all NAADP producing enzymes have been thought to be members of the ADP-ribosyl cyclase family. ADP-ribosyl cyclases exhibit promiscuous substrate selectivity, synthesize a variety of products and are regulated in a limited manner, which may be non-physiological. We now report the presence of an enzyme on the surface of sea urchin sperm that exhibits bell-shaped regulation by Ca 2+} over a range (EC 50} of 10 nM and IC 50} of 50 µM) that is physiologically relevant. Uniquely, this surface enzyme possesses complete selectivity for nucleotides with a 2'-phosphate and exhibits only base-exchange activity without any detectable cyclase activity. Taken together, these findings indicate that this novel enzyme should be considered as the first true NAADP synthase.
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