THE SENSITIVITY OF BOVINE CORNEAL EPITHELIAL LYSO-PAF ACETYLTRANSFERASE TO CYCLOOXYGENASE AND LIPOXYGENASE INHIBITORS IS INDEPENDENT OF ARACHIDONATE METABOLITES

Lyso-platelet-activating factor (PAF) acetyltransferase is a critical regulatory step in PAF synthesis. PAF accumulates in the cornea in res ponse to injury and is a potent inflammatory mediator that stimulates corneal cyclooxygenase but not lipoxygenase reactions. 12(S)-hydroxyei cosatetraenoic (HETE) acid, a major lipoxygenase product of mammalian corneas, is also generated during injury. In the bovine corneal epithe lium, both PAF acetyltransferase and 12-lipoxygenase are microsomal en zymes. A potential interaction between these two lipid mediators was, therefore, examined. PAF acetyltransferase activity was assayed by det ermining radioactivity in the trichloroacetic acid-precipitated comple x of [H-3]PAF bound to albumin, formed after incubation of corneal epi thelial microsomes with lyso-PAF and [H-3]acetyl CoA. Lipoxygenase met abolism by bovine corneal epithelial microsomes was also studied. Whil e 12(S)-HETE did not activate lyso-PAF acetyltransferase, PAF synthesi s was decreased when microsomes were treated with lipoxygenase inhibit ors. The IC50 values for nordihydroguaiaretic acid (NDGA), and baicale in were 75 and 105 mu M, respectively. The IC50 value for CDC, a more potent inhibitor of platelet 12-lipoxygenase, was greater than 200 mu M. The results indicate that concentrations suppressing lyso-PAF acety ltransferase activity exceed those required to inhibit lipoxygenases f rom bovine corneal epithelia. Similarly, concentrations of aspirin and indomethacin, cyclooxygenase inhibitors that decrease PAF formation, were greater than those reported to block prostaglandin generation. A number of other compounds, some structurally similar to the lipoxygena se inhibitors that suppress lyso-PAF acetyltransferase, and others unr elated chemically but known as anti-oxidant and cationic-chelators, al so inhibited lyso-PAF acetyltransferase. This suggests that lyso-PAF a cetyltransferase activity is inhibited by mechanisms independent of li poxygenase and cyclooxygenase metabolites.