Nicotinic acid adenine dinucleotide phosphate
, also known as NAADP<sup>+</sup>
, is a nucleotide very similar to NADP
, with the nicotinamide
replaced with a niacin
molecule. NAADP is one of the most potent Calcium signalling
messenger molecules, causing release of calcium from intracellular stores.
Bifunctional ectoenzymes of the CD38
family is believed to synthesize NAADP<sup>+</sup> from NADP
<sup>+</sup> (by exchange of nicotinamide
under acidic conditions). However whether the so called base exchange reaction is responsible for NAADP generation in vivo, is questionable and remains to be proven. NAADP is believed to be hydrolysed in two selective mechanism. 1. Hydrolysis by CD38 and 2'phosphatase cleavage by a Ca2+ sensitive phosphatase. Theoretically pyrophosphatase also have the capability to hydrolyse NAADP.
The two paralogous enzymes- transmembrane CD38
and GPI anchored CD157
, that produce NAADP (and cADPR) in humans both have their active synthesis site in the ectodomain. Though this may involve vesicular synthesis but it has been shown that it is produced at the extracellular sites, and also can act when produced by a different cell or added artificially from outside. So the NAADP has to enter the cell either by diffusion or by transport. Considering the fact that the substrate of NAADP synthesis (NADP) itself is very sparse in the extracellular medium, a purse diffusion based mechanism has been... Read More