The effect of heart and skeletal muscle troponin complexes and calmodulin on the Ca2+-dependent reactions of phosphorylase kinase isoenzymes.

The dephosphorylated form of phosphorylase kinase was purified 700-fold from rabbit heart extract. The purified enzyme had a pH 6.8/pH 8.2 activity ratio of 0.04-0.08 and was completely dependent on Ca2+ with an apparent Ka value for Ca2+ of 2.59 microM at pH 6.8. At free Ca2+ concentrations between 0.057 microM and 400 microM, 1.5 microM rabbit heart troponin complex had no significant effect on the reaction. However, 1.5 microM rabbit skeletal muscle troponin complex stimulated the reaction 1.5-2-fold with a concomitant decrease in the Ka value for Ca2+ to 1.40 microM. No differences in the effects of these troponin complexes were observed when heart-type and skeletal muscle-type phosphorylase b isoenzymes from either rabbit or pig were used as substrate. Similar effects of heart and skeletal muscle troponin complexes were observed on the Ca2+-dependent reaction of the dephosphorylated form of phosphorylase kinase partially purified from rabbit skeletal muscle. A saturating concentration (1.36 microM) of bovine brain calmodulin stimulated 2-5-fold the Ca2+-dependent reaction of skeletal muscle phosphorylase kinase, but not the reaction of heart phosphorylase kinase. Heart troponin complex (12 microM) suppressed 80-100% the stimulatory effect of skeletal muscle troponin complex on the reactions of phosphorylase kinase isoenzymes, but had no significant effect on the stimulation by calmodulin of skeletal muscle phosphorylase kinase reaction.