Dissimilar protection of tocopherol isomers against membrane hydrolysis by phospholipase A2

Porcine pancreatic phospholipase A(2) (PLA(2)) hydrolyses phosphatidylcholine when in the lamellar state as well as in the micellar state. We have found that alpha-tocopherol, the most active form of vitamin E, is able to inhibit PLA(2) activity only toward lamellar fluid membranes, thus protecting phospholipids toward this lytic enzyme. This compound decreases both the initial rate and the extent of hydrolysis. The inhibition is of the non-competitive type and the evidence strongly suggests that it is due to an effect of alpha-tocopherol on the substrate, i.e. the membrane, and not on the enzyme itself. Other tocopherols, such as the isomers beta-, gamma- and delta-tocopherol also display PLA(2) inhibition but consecutively to a lower extent. The grade of inhibition of PLA(2) activity by tocopherols correlates well with their biological activity and with their location in the bilayer as shown by fluorescence quenching. Cholesterol does not inhibit PLA(2) activity at concentrations even higher than those of tocopherols, indicating that the effect of tocopherols is not due to alteration of membrane fluidity. The possible mechanisms underlying the different behaviour of tocopherol isomers as PLA(2) inhibitors are discussed considering its biological significance as membrane stabilizers, suggesting biological actions of compounds with vitamin E activity other than their classical roles as antioxidants. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.