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 extra cts 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-Ment en constant (K-m) values and enzymatic mechanisms, as well as distinct sens itivities to inhibitors, oxidants, reductants and CaCam inhibitors. It is t herefore 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, po ssessing 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.