INHIBITION OF HUMAN SKIN PHOSPHOLIPASE-A(2) BY LIPOCORTINS IS AN INDIRECT EFFECT OF SUBSTRATE LIPOCORTIN INTERACTION

Proteins of the annexin/lipocortin family have been claimed to mediate the anti-inflammatory action of glucocorticosteroids by the inhibitio n of phospholipases A2. This hypothesis has been challenged by the fin ding that annexins do not directly interact with the enzyme in a class ical enzyme/inhibitor behavior, but more likely block the access of th e phospholipase A2 to its substrate by binding to phospholipids. Becau se former studies with skin phospholipase A2 Suggested a specific regu lation by annexin-1, we investigated the substrate dependence of this effect. For this purpose phospholipase A2 activities in human epidermi s and dermis homogenates were measured in the presence of various amou nts of annexins-1,2, or -5. The respective annexin was preincubated in separate series either with the substrate or with the enzyme. We foun d a partial inhibition of both epidermal and dermal phospholipase A2 a ctivities with all annexins tested (annexin-5 much greater than annexi n-2 > annexin-1). The inhibitory effect was absolutely dependent on th e annexin/phospholipid ratio and occurred only at very high annexin co ncentrations relative to the amount of substrate. Our data demonstrate that the inhibition of human skin phospholipase A2 by annexins depend s on the substrate concentrations, as has been shown for phospholipase s A2 of other origins as well. All observations can be explained by th e current ''substrate depletion model'' characterizing the indirect ef fects of annexins on phospholipase A2 activities. It is therefore rath er unlikely that annexins are directly involved in the regulation of p hospholipase A2 activity of human skin under physiologic conditions.