Myocardial protein kinases: I. Properties of soluble and membrane-bound catalytic subunits

Abstract

The particulate fraction of rat left ventricle, under certain specific assay conditions, was found to contain as much protein phosphotransferase activity as the soluble fraction. Both the soluble and the membrane-bound holoenzyme could be activated by cyclic AMP. The properties of the catalytic subunit of the soluble enzyme differed from those of the catalytic subunit of the particulate fraction with respect to substrate specificity and apparent K M values for ATP. The soluble enzyme was found to have a very low apparent K M for ATP (5.0 × 10 −6 m) and was inhibited by ATP concentrations above 0.2 m m. The particulate enzyme, on the other hand, showed a high apparent K M for ATP (1.3 × 10 −4 m) and the activity increased with increasing ATP concentrations up to 0.4 m m. The activities in the soluble and particulate fractions were observed to vary inversely between ATP concentrations of 0.1 m m and 0.4 m m. It is concluded that ATP concentration may play a role in controlling the activity of the catalytic subunit after its dissociation from the holoenzyme (by cyclic AMP binding to the regulatory subunit).