Effect of beta 2-glycoprotein I on the activity of adenylate cyclase in platelet membranes

Washed human platelets were preincubated with 0-300 micrograms/ml human beta 2-glycoprotein I and the effect of this on the adenylate cyclase activity (EC.4.6.1.1.) was studied. Adenylate cyclase activity could be increased 2-3 fold. The same degree of activation was seen when low concentrations of prostaglandin E1 (1 microM) had been present concomitant with beta 2-glycoprotein I during preincubation. The dose-response curves of the adenylate cyclase activity measured as a function of the beta 2-glycoprotein I concentration were S-shaped in the absence of prostaglandin E1 and hyperbolic in its presence. The results suggest a biological function of beta 2-glycoprotein I as a compound conserving and activating the membrane-bound adenylate cyclase.