Subunit structure of lysine sensitive aspartate kinase from spinach leaves

The lysine-sensitive isoenzyme of aspartate kinase was purified to homogeneity from spinach leaves and its subunit composition was studied. The purified preparation had an apparent molecular mass of 280,000 and separated into two subunits- a large subunit with molecular mass of 53,000 and smaller subunit with molecular mass of 17,000 by urea treatment and SDS PAGE. The enzyme molecule has subunit composition of 4 large and 4 small subunits. The activity of the large subunit was stimulated more than two fold by the addition of small subunit and the stimulated activity was inhibited by EGTA. This inhibition could be reversed by Ca++. Further characteristics of the smaller subunit such as heat stability, behavior on ion exchange chromatography, elctrophoretic mobility on polyacrylamide gels, amino acid composition and pattern, presence of trimethyl lysine, its ability to activate other calmodulin stimulated enzymes and its calmodulin-like nature in RIA tests suggested that this subunit is identical to calmodulin.