Calmodulin-dependent and independent NAD kinase activities from cytoplasmic and chloroplastic fractions of spinach (Spinacia oleracea L.)

Abstract

NAD kinase activity has been found in a soluble, cytoplasmic fraction and in the chloroplasts prepared from green spinach leaves. A small amount of both the cytoplasmic and the chloroplastic NAD kinase activities was retained on a calmodulin-Sepharose affinity column. The cytoplasmic NAD kinase eluted from the affinity column was found to be enhanced by calmodulin in a Ca²⁺-dependent manner. The chloroplastic enzyme which is located exclusively in the stroma and not in the envelope and thylakoid fractions was not affected by Ca²⁺ and calmodulin. The stromal fraction of purified chloroplasts contained only a negligible amount of calmodulin, most probably due to cytoplasmic contamination. Based on these data, two different mechanisms for the light-dependent modulation of spinach NAD kinase activity are suggested.