Effects of alpha-tocopherol, its carboxylic acid chromane compound and two novel antioxidant isoflavanones on prostaglandin H synthase activity and autodeactivation.

The natural antioxidant alpha-tocopherol has repeatedly been described to inhibit platelet aggregation and thromboxane formation, whereas its influence on prostaglandin H synthase in vivo and in vitro is a matter of controversy. In the present study the effects of different antioxidative compounds on ram vesicular gland microsomal prostaglandin H synthase activity were investigated in vitro: d,l-alpha-tocopherol, its carboxylic acid chromane compound (Trolox), phytol, alpha-tocopherol-acetate and two novel antioxidative isoflavanones, obtained by methylation and/or hydrogenation of naturally occurring isoflavones from fermented soybeans (6,7-dihydroxy-4'-methoxyisoflavanone and 6,7,4'-trihydroxyisoflavanone). Alpha-tocopherol, -acetate and phytol revealed no significant influence on the enzyme activity when applied in concentrations up to 1 mM. Trolox (100-1000 mu mumol/l) and the two isoflavanones (5-50 and 10-100 mumol/l) dose-dependently augmented the initial rate of oxygen consumption and the total oxygen uptake during prostaglandin H synthase incubation with arachidonic acid (AA). In parallel, these compounds increased the formation of prostaglandin E2 and F2 alpha from 14C-labelled AA, and they markedly protected the prostaglandin H synthase from rapid autodeactivation as revealed by repetitive application of AA in small doses. We suggest that these compounds serve as cosubstrates to which the oxidizing equivalents are transferred which arise during the hydroperoxidase reaction of the enzyme.