Phosphorylation of cardiac troponin inhibitory subunit (troponin I) and tropomyosin-binding subunit (troponin T) by cardiac phospholipid-sensitive Ca2+-dependent protein kinase

Abstract

Cardiac [bovine] phospholipid-sensitive Ca2-dependent protein kinase phosphorylated cardiac troponin inhibitory subunit (troponin I) and tropomyosin-binding subunit (troponin T), present either as the free form or the troponin-tropomyosin complex. Exhaustive phosphorylation of troponin I and of troponin T revealed that 1.7 and 2 mol of phosphate was incorporated per mol of the subunits, respectively. cAMP-dependent protein kinase, though incorporating 0.8 ml of phosphate per mol of troponin I, was unable to phosphorylate troponin T. Phosphorylation of troponin I (apparent Km = 3.4 .mu.M; Vmax. = 2.6 .mu.mol/min per mg of enzyme) or troponin T (apparent Km = 0.3 .mu.M; Vmax. = 0.5 .mu.mol/min per mg of enzyme) by the Ca2+-dependent enzyme was inhibited by various agents, such as adriamycin, palmitoylcarnitine, trifluoperazine, melittin and N-(6-aminohexyl)-5-chloronaphthalene-1-sulfonamide (compound W-7). Ca2+ antagonists (such as verapamil), forskolin and ouabain were ineffective. Apparently, troponin I and troponin T were effective substrates for this species of Ca2+-dependent protein kinase, suggesting its potential regulatory role in the contractile activity of myofibrils modulated by troponin.