Purification and characterization of two protein kinases from bovine heart mitochondrial membrane

Two protein kinases (MPK1 and MPK2) were isolated from bovine heart mitochondria. After the solubilization of submitochondrial particles with cholate, these protein kinases were purified by ammonium sulfate precipitation, Sepharose 6B gel filtration, and affinity chromatography on phosvitin-Sepharose. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the final preparation of MPK1 contained two major polypeptide bands (40,000 and 36,000 daltons). MPK2 contained one major polypeptide of 34,000 daltons. Under nondenaturing conditions, the molecular weights of MPK1 and MPK2 were estimated to be approximately 250,000 and 70,000-90,000, respectively. MPK1 had a pH optimum at 9.0 and MPK2 at 7.5. Both enzymes required Mg2+ for activity and responded poorly to Mn2+ or Ca2+. Both had similar apparent Km values for ATP and were not affected by either cyclic AMP or Ca2+-calmodulin. MKP1 phosphorylated threonine residues and MPK2 serine residues of casein. With beta casein as substrate, MPK1 was more active than MPK2, whereas with alpha casein, MPK2 was more active than MPK1.