KINETIC AND THERMODYNAMIC PROPERTIES OF MEMBRANE-BOUND CA-ATPASE WITHLOW-AFFINITY TO CALCIUM IN CARDIAC SARCOLEMMA - RESPONSE TO GLOBAL-ISCHEMIA OF THE HEART

Thermodynamic and kinetic properties of membrane bound Ca-ATPase with low affinity to calcium in cardiac sarcolemma were investigated with r espect to the effect of global ischemia on the heart. Energy barrier f or ATP hydrolysis catalyzed by the Ca-ATPase was slightly higher in he arts subjected to ischemia, as it was evident from increased values of activation energy. Ischemia also induced a time dependent decrease in the activity and maximum velocity (Vmax) value of Ca-ATPase. The depr ession of enzyme activity was evident as early as 15 minutes after the onset of ischemia. After 30 minutes of ischemia the decrease in Vmax slowed down, probably due to an ''adaptational'' decrease of the Km va lue for Ca-ATPase. This phenomenon may be interpreted as a mechanism b y which the enzyme attempts to keep working in a situation when the su pply of ATP is insufficient.