Dephosphorylation of the Inhibitor Component of Troponin by Phosphorylase Phosphatase

Phosphorylase phosphatase from rabbit skeletal muscle, when purified 700-fold, dephosphorylated the 22,000 molecular weight subunit of troponin (TNI). The endogenous phosphate in TNI and that incorporated by phosphorylase kinase were both totally removed at identical rates. Five criteria were established to show that dephosphorylation of TNI was catalyzed by phosphorylase phosphatase and not a contaminant in the preparation: (a) co-purification of TNIand phosphorylase-phosphatase activities; (b) identical patterns of the activities on polyacrylamide gel electrophoresis; (c) parallel heat denaturation and (d) fluoride inhibition of the activities; (e) competitive effects when both TNI and phosphorylase a were present. The Km for phosphorylase a was 6.8 µm, and 21.2 µm for TNI. The Vmax with TNI was approximately 4-fold greater than with phosphorylase a.