Inactivation of phospholipase A2 by naturally occurring biflavonoid, ochnaflavone

Ochnaflavone, a medicinal herb product isolated from Lonicera japonica, strongly inhibited rat platelet phospholipase A2 (IC50, about 3 microM). 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 A2 than group I phospholipase A2 (IC50, about 20 microM). Addition of excess Ca2+ concentration up to 8 mM did not antagonize the inhibitory activity of ochnaflavone. These results indicate that the inhibition of phospholipase A2 by ochnaflavone may result from direct interaction with the enzyme.