Presence of a Ca2+-sensitive CDPdiglyceride-inositol transferase in canine cardiac sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) and plasma membranes from canine left ventricle were used to evaluate the presence of the enzyme CDPdiglyceride-inositol transferase in these membranes. (K+,-Ca2+)-ATPase activity, a marker for SR, was 79.2 +/- 5.0 (SE) and 11.2 +/- 2.0 μmol x mg-1 x h-1 in SR and plasma membrane preparations, respectively, and (Na+, K+)-ATPase activity, a marker for plasma membranes, was 5.6 +/- 1.2 and 99.2 +/- 8.0 μmol x mg-1 x h-1, respectively. Contamination of SR and plasma membrane preparations by mitochondria was estimated to be 2% and 8%, respectively, and by Golgi membranes, 0.9% and 1.8%, respectively. The transferase activity detected in the plasma membrane preparation could be accounted for largely, but not entirely, by contaminating SR membranes. The pH optimum for the SR transferase activity was between 8.0 and 9.0. Ca2+ inhibited the enzyme, half-maximal inhibition occurring at about 10 μM Ca2+. No loss of [3H]PtdIns could be detected when membranes were incubated in the presence or absence of Ca2+. The Ca2+ inhibition of the transferase was noncompetitive with respect to CDP-dipalmitin while that with respect to myo-inositol was slightly noncompetitive at low [Ca2+] and became uncompetitive at higher [Ca2+]. It is concluded that CDPdiglyceride-inositol transferase is present on SR membranes and is sensitive to micromolar Ca2+. The data are consistent with a putative role for the inhibition of the SR transferase by Ca2+ and acidic pH in the protection of the SR against calcium overload in ischemic myocardium