Formation of leukotrienes C and D and Pharmacologic modulation of their synthesis

Our attempts to find a physiologically relevant means for inducing leukotriene synthesis in rat peritoneal mononuclear cells have thus far been unsuccessful in addition to the IgE-anti-IgE challenge which we previously reported, we have now tried human C3a and C5a as well as crude and semipurified fractions of the prostaglandin-generating factor of anaphylaxis. In each case, it was possible to show that these substances activated the cells even though no leukotrienes were formed. A cell-free system in which LTC + LTD formation can be studied was developed as a modification of published methods. Arachidonic acid and LTA4 served as precursors in this system in the presence of added glutathione. Calcium was required for LTC and LTD synthesis from arachidonic acid but was not required for the glutathione S-transferase terminal step in the synthesis. Using inhibitor profiles, substrate specificity for chromogenic substrates, and inactivation by selective antibodies, we tried to identify the subtype of the glutathione S-transferase in RBL cells. Although antibody to type E of the enzyme was most effective in neutralizing the enzyme activity, neither the substrate specificity nor the inhibition profiles agreed with the conclusion that the E-type enzyme was the major form in these cells. The effect of known inhibitors of glutathione S-transferase on the conversion of arachidonate to LTC and LTD was examined. Bilirubin, an inhibitor which binds to the enzyme and is not a substrate, was much more active in inhibiting LTC + LTD formation than were steroid sulfates, which were markedly less active in inhibiting this reaction. The inhibitory activities of the other compounds were similar on all substrates tested.