INACTIVATION OF PHOSPHOLIPASE A(2) BY NATURALLY-OCCURRING BIFLAVONOID, OCHNAFLAVONE

Ochnaflavone, a medicinal herb product isolated from Lonicera japonica, strongly inhibited rat platelet phospholipase A(2) (IC50, about 3 mu M). Inactivation was concentration and pH dependent (maximum inactivation occurred between pH 9.0 and 10.0). Ochnaflavone inhibited the enzyme by a noncompetitive manner, with the apparent Ki value of 3 x 10(-5) M. Reversibility was studied directly by dialysis method; the inhibition was irreversible. In addition, the inhibitory activity of ochnaflavone is rather specific against group II phospholipase A(2) than group I phospholipase A(2)(IC50, about 20 mu M). Addition of excess Ca2+ concentration up to 8mM did not antagonize the inhibitory activity of ochnaflavone. These results indicate that the inhibition of phospholipase A(2) by ochnaflvone may result from direct interaction with the enzyme. (C) 1994 Academic Press, Inc.