Creatine kinase is the main target of reactive oxygen species in cardiac myofibrils

Reactive oxygen species (ROS) have been reported to alter cardiac myofibrillar function as well as myofibrillar enzymes such as myosin ATPase and creatine kinase (CK). To understand their precise mode and site of action in myofibrils, the effects of the xanthine/xanthine oxidase (X/XO) system or of hydrogen peroxide (H2O2) have been studied in the presence and in the absence of phosphocreatine (PCr) in Triton X-100-treated cardiac fibers. We found that xanthine oxidase (XO), with or without xanthine, induced a decrease in maximal Ca2+-activated tension. We attributed this effect to the high contaminating proteolytic activity in commercial XO preparations, since it could be prevented by a protease inhibitor, phenylmethylsulfonyl fluoride (PMSF), and it could be mimicked by trypsin. In further experiments, XO was retreated with 1 mmol/L PMSF. Superoxide anion production by the X/XO system, characterized by electron paramagnetic resonance spin-trapping technique, was not altered by PMSF. A slight increase in maximal force was then observed either with X/XO (100 μmol/L per 30 mIU/mL) or H2O2. pMgATP- rigor tension relationships have been established in the presence and in the absence of PCr to separate the effects of ROS on myosin ATPase and myofibrillar-bound CK. In the presence of PCr, pMgATP50, the pMgATP necessary to induce half-maximal rigor tension, was reduced from 5.03±0.17 (n=21) to 4.22±0.22 (n=4) after 25 minutes of incubation in the presence of 30 mIU/mL XO and 100 μmol/L xanthine or to 4.04±0.1 (n=11) after incubation in the presence of 2.5 mmol/L H2O2. The ROS effects were partially prevented or antagonized by 1 mmol/L dithiothreitol. No effect was observed on pMgATP50 when PCr was absent. pCa-tension relationships have been evaluated to assess the effects of ROS on active tension development. Incubations with H2O2 induced an increase in Ca2+ sensitivity and resting tension when MgATP was provided through myofibrillar CK (PCr and MgADP as subtrates) but not when MgATP was added directly. These results suggest that myofibrillar CK was inhibited by ROS. Active stiffness and the time constant of tension changes after quick stretches applied to the fibers were dose- dependently increased by H2O2 only in the presence of PCr. In addition, myofibrillar CK but not myosin ATPase enzymatic activity was depressed after incubation with either ROS. These results suggest that ROS mainly alters CK in myofibrils, probably by the oxidation of its essential sulfhydryl groups. Such CK inactivation results in a decrease in the intramyofibrillar ATP-to- ADP ratio. The effects of ROS on cytosolic and bound CKs may take part in the overall process of myocardial stunning after cardiac ischemia and reperfusion.