Phospholipase D: A New Avenue to the Phospholipid Signalling Pathways in the Myocardium. In: Ostadal B, Dhalla NS, editors. Heart Function in Health and Disease. Boston: Springer US; 1993. pp. 79-90. [DOI: 10.1007/978-1-4615-3090-9_7]

Kinetics of myocardial phospholipase D

Myocardial phospholipase D (PLD) is located in different subcellular membranes, including sarcolemma (SL) and sarcoplasmic reticulum (SR). In this study, the kinetics of PLD-dependent hydrolytic and transphosphatidylation activities were examined in SL and SR fractions isolated from rat heart by measuring the formation of phosphatidic acid and phosphatidylethanol, respectively. The results showed that, compared to SR PLD, SL PLD had a higher Vmax, i.e. 373 vs. 70 nmol/mg protein/h for the hydrolytic activity and 415 vs. 60 nmol/mg protein/h for the transphosphatidylation activity. In comparison with the SR enzyme, SL PLD had a lower Km value for the hydrolytic activity (0.46 vs. 0.65 mM), buy a higher Km for the transphosphatidylation activity (225 vs. 179 mM). These distinctive kinetic parameters suggest that SL PLD and SR PLD may be isoforms of the enzyme and/or have different membrane domain. Therefore, SL- and SR-localized PLD activities may be under independent control mechanism(s) and play distinct roles in normal conditions and pathological processes.

Depression of cardiac sarcolemmal phospholipase D activity by oxidant-induced thiol modification

Myocardial phospholipase D (PLD) is primarily localized at the sarcolemmal level and selectively hydrolyzes phosphatidylcholine to form phosphatidic acid as part of the signal transduction mechanisms for regulating Ca2+ movements in the heart. Since the myocardial cell damage induced by oxidative stress is associated with abnormalities in Ca2+ homeostasis and thiol status, we examined the thiol group dependence and the effects of oxidant species on this enzyme. Sarcolemmal membranes isolated from rat heart were exposed to several types of thiol group modifiers. Alkylation with N-ethylmaleimide or methyl methanethiosulfonate, mercaptide formation with p-chloromercuriphenylsulfonic acid, and thiol-disulfide exchange with 5,5'-dithio-bis(2-nitrobenzoate) depressed sarcolemmal PLD activity; in all cases the depression was prevented by dithiothreitol. At different concentrations of N-ethylmaleimide the PLD depression correlated well (r = 0.98) with the decrease in total thiol group content of the membrane. The enzyme activity was not affected by xanthine-xanthine oxidase, a superoxide anion-generating system, but was depressed by hydrogen peroxide (H2O2) in a concentration-dependent manner. This inhibitory effect was prevented by catalase as well as by dithiothreitol, but not by D-mannitol. The effect of a hydroxyl radical-generating system (Fenton reaction) could not be assessed because of an interfering direct inhibition by Fe2+. Dithiothreitol was also able to restore PLD activity in H2O2-pretreated membranes and to prevent a severe deactivation of the enzyme by hypochlorous acid (HOCI). Protection by glutathione and inhibition by its oxidized form were also observed.(ABSTRACT TRUNCATED AT 250 WORDS)