Temperature-dependent deacylation of molecular species of phosphatidylcholine by microsomal phospholipase A2 of thermally acclimated rainbow trout, Salmo gairdneri.
Using the ratios of kinetic parameters, V/Km, the deacylation of different molecular species of 1-palmitoyl,2-acyl phosphatidylcholine via microsomal phospholipase A2 (PLA2) was studied in liver tissue of thermally acclimated rainbow trout (Salmo gairdneri). In general, PLA2 from fish acclimated to cold temperatures showed an order of preference for the acyl moieties of 18:1 greater than 18:2 greater than 18:0. Trout acclimated to warm temperatures generally preferred 18:0 PC, but the actual order of preference depended on the temperature of the assays and the presence of endogenous lipids in the enzyme preparation. At 5 C, the particulate (microsomal) enzyme preferred 18:0 greater than 18:2 greater than 18:1, but a lipid-free preparation of the enzyme preferred 18:2 greater than 18:0 greater than 18:1. At 20 C, particulate enzyme preferred 18:1 greater than 18:0 greater than 18:2 but purified enzyme preferred 18:0 greater than 18:2 greater than 18:1. Thus, assay temperature and the presence of microsomal lipids had a greater effect on PLA2 from fish acclimated to warm temperatures than fish acclimated to cold temperatures. The substrate preference of PLA2 is discussed with reference to the previously observed changes in membrane fatty acid composition that occur with thermal acclimation in rainbow trout.