Ca2+- and Phospholipid-Independent Activation of Protein Kinase C by Selective Oxidative Modification of the Regulatory Domain

The susceptibility of purified protein kinase C to oxidative inactivation by H2O2 was found to be increased by Ca2+ either alone at a high (5 mM) concentration or at a low (≈ 50 μ M) concentration along with phosphatidylserine and diacylglycerol and by tumor-promoting phorbol esters even in the absence of Ca2+. This suggested that the membrane-bound and/or catalytically active form of protein kinase C is relatively more susceptible to oxidative inactivation. Although both the regulatory and catalytic domains of protein kinase C were susceptible to oxidative inactivation, a selective modification of the regulatory domain was obtained under mild oxidative conditions by protecting the catalytic site with ATP/Mg2+. Under these conditions there was a loss of both phorbol ester binding and Ca2+/phospholipid-stimulated kinase activity. However, this modified form of enzyme exhibited an increase in Ca2+/phospholipid-independent kinase activity. This suggests that selective oxidative modification of the regulatory domain may negate the requirement for Ca2+ and lipids for activation. Treatment of intact C6 glioma or B16 melanoma cells with H2O2 resulted in a time- and temperature-dependent decrease in Ca2+/phospholipid-dependent protein kinase C activity along with a concomitant transient increase in an oxidatively modified isoform of protein kinase C that exhibited activity in the absence of Ca2+ and phospholipids. Since protein kinase C can initially be activated by mild oxidative modification and subsequently inactivated by further oxidation, this dual activation-inactivation of protein kinase C in response to H2O2 suggests an effective on/off signal mechanism to influence cellular events.