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

Abstract

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 (K_m) 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 K_m values corresponding to the intracellular concentrations of ATPMg and NAD⁺. We speculate that the 63-kDa CaCam-dependent NAD⁺ kinase, possessing a low K_m, 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.