I know that NGD is catalysed by CD38, CD157 and Aplysia cylase to produce the hydrolysis-resistant cGDPR (Fernàndez et al., 1998; Graeff et al., 1994; Guse et al., 2001; Lee et al., 1999; Zocchi et al., 2001), however:
Does NGD (added in surplus) act as a competitive antagonist for NAD cyclisation at these enzymes? My understanding is that there is a common active site for the cyclisation of both these substrates (Lee et al. 1999). Such competitive antagonism (Ki = 24 μM) has been shown for a non-CD38 "synaptosomal" cyclase. However, although this enzyme can bind NGD it cannot metabolise it (Ceni et al., 2006, 2003). Antagonism ustilising NGD (2 mM) has also been shown for Ach-mediated Ca2+ responses in permeabilised myocytes, which are dependent on a signalling cascade involving muscarinic M2 receptors, ARC (likely CD38), FKBP12.6 and RyR2 (Fritz et al., 2005). cGDPR does not cause calcium mobilisation (Graeff et al., 1996), but does it still compete with cADPR at a RyR-stimulating active site?If the constitutively active ARC in my cells of choice (cultured neurons) is intracellular (as opposed to a potentially extracellular active site), is there a transport mechanism which would give extracellularly applied NGD access to the active site? Potential candidates in my mind are: a) its internalisation with CD38 (Zumaquero et al., 2010), b) transport through a proposed CD38 (hydrophilic) pore (De Flora et al., 2004; Franco et al., 1998), nucleoside transporters (as characterised for cADPR; Guida et al., 2004, 2002) or P2X7Rs (as characterised for NADH/NAD; Lu et al., 2007; Salmina et al., 2014).Anything that could add to my picture of the use of this compound would be greatly appreciated.
Thanks for your time,
Aidan
(PhD candidate)
