Characterization of soluble calcium calmodulin-dependent and -independent NAD+ kinases from Avena sativa seeds

Two NAD+ kinase isoforms were separated from Avena sativa L. embryo extracts from dormant and after-ripened seeds by size-exclusion chromatography: a 410-kDa Ca2+–calmodulin (CaCam)-independent NAD+ kinase was observed in both dormant and after-ripened seeds and a 63-kDa CaCam-dependent isoform was found to be abundant only in embryos of after-ripened Avena seeds. The characterization of these two isoforms showed differences in Michaelis–Menten constant (Km) values and enzymatic mechanisms, as well as distinct sensitivities to inhibitors, oxidants, reductants and CaCam inhibitors. It is therefore proposed that the 410-kDa isoform could be a ‘housekeeping’ enzyme with Km values corresponding to the intracellular concentrations of ATPMg and NAD+. We speculate that the 63-kDa CaCam-dependent NAD+ kinase, possessing a low Km, would be mainly involved in the adaptation and response of A. sativa to environmental signals or stresses through changes of redox potential and/or calcium signalling pathways.