Isolation, chacterization and regulation of isoenzymes of aspartate kinase differentially sensitive to calmodulin from spinach leaves☆

Abstract Two isofunctional forms of aspartate kinase, one sensitive to lysine (isoenzyme I) and the other to threonine (isoenzyme II) have been isolated from spinach leaves by DEAE- and Sephadex G-200 column chromatography with molecular weights of 330 000 and 200 000, respectively. These forms differ also in their sensitivity to Ca 2+ and calmodulin. While the form inhibited by lysine is insensitive to Ca 2+ and calmodulin, the form that is inhibited by threonine is activated 3–4-fold by calmodulin. The effect fo calmodulin is furtehr increased by Ca 2+ and is inhibited by EGTA, which specifically chelates out Ca 2+ . This inhibition can be relieved by Ca 2+ . The apparent K m values for asparatate and ATP in the case of the lysine-sensitive isoenzyme, isoenzyme I, which is unaffected by calmodulin, also remain unaffected by calmodulin. However, the apparent K m values for the two substrates are reduced in the presence of calmodulin in the case of the threonine-sensitive isoenzyme, isoenzyme II.