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Human placental cytosol inhibits platelet aggregation induced by high doses of collagen. The aim of this study was to investigate whether this anti-aggregating activity was caused only by the presence of various activities already described in the placenta (an ADP-consuming enzyme, a fatty acid cyclooxygenase inhibitor, and a thromboxane synthetase inhibitor) or whether another factor was present. Heating the cytosol at 50 degrees C for 6 min destroyed the inhibitor of collagen-induced aggregation. ADPase and the AA pathway inhibitors were not modified by this treatment. We therefore show the presence of an additional anti-aggregating factor: it is destroyed by heating at 50 degrees C. We also tested for the presence of an inhibitor of AA release in the placental cytosol using three different methods (rabbit platelets in PRP, washed rabbit platelets, and NRK fibroblasts) but no inhibition could be evidenced. We conclude that this new anti-aggregating factor, which is probably a protein, acts neither through AA release inhibition nor AA cascade inhibition.

It has been shown that various glutathione transferases can synthesize leukotriene C4, or its methyl ester, from glutathione and leukotriene A4. We questioned whether the same enzymes could be used to resolve racemic leukotriene A4 methyl ester (more easily prepared than the optically active enantiomer) and to produce leukotriene C4 methyl ester selectively. We present in this paper a study of the enantioselectivity of some rat liver glutathione transferase isozymes and of the glutathione transferase of human placenta for the leukotriene A4 methyl ester isomers. The rat liver 3-4 glutathione transferase exhibited the highest conversion rate but preferentially converted the (5R, 6R) leukotriene A4 methyl ester. The placental enzyme was fairly selective for the natural (5S, 6S) enantiomer but the rate of conversion was low.