Tocopherol analogs suppress arachidonic acid metabolism via phospholipase inhibition

α-Tocopherol and three derivatives in which the phytol chain is modified or deleted were examined for their effect on cultured keratinocyte arachidonic acid metabolism. 2,2,5,7,8-Pentamethyl-6-hydroxychromane (PMC), in which the phytol chain is replaced by a methyl group, inhibited basal, bradykinin (BK)- and A23187-stimulated prostaglandin E₂ (PGE₂) synthesis with an apparent K_i of 1.3 μM. The K_i of the analogue with six carbon atoms in the side chain (C6) was 5 μM while that of the C11 analogue was 10 μM. No effect of α-tocopherol was observed. The mechanism of inhibition was studied using PMC. The effect of PMC on phospholipase and cyclooxygenase activity was assayed using stable isotope mass measurements of PGE₂ formation, which assesses arachidonate release and cyclooxygenase metabolism simultaneously. BK-stimulated formation of PGE₂, derived from endogenous phospholipid, was decreased 60% by 5 μM PMC and eliminated by 50 μM PMC, compared with controls. No difference in PGE₂ formed from exogenous arachidonic acid was observed, indicating no effect of PMC on cyclooxygenase activity. In contrast, no effect of 5 μM PMC was observed on BK-stimulated [³H]arachidonic acid release from prelabeled cultures. The capacity of PMC to inhibit phospholipase activity in vitro was also assessed. PMC inhibited hydrolysis of phospholipid substrate by up to 60%. These results suggest that α-tocopherol analogues with alterations in the phytol chain inhibit eicosanoid synthesis by preferential inhibition of phospholipase.