Inhibition of collagen-induced platelet aggregation by normal plasma.

Normal plasma has been found to inhibit the platelet aggregation-inducing effect of collagen in a time consuming reaction independent of temperature. Collagen treated with serum and washed has reduced reactivity which can be restored to normal by treatment with 1.5 M sodium chloride. On the basis of this result, it is suggested that inhibition results from adsorption to collagen of a plasma component. The inhibitory plasma component is destroyed at 56 degrees C, is unstable below pH 7, and migrates with the alpha globulins on starch block electrophoresis at pH 8.6. On the basis of ultrafiltration and sucrose density gradient ultracentrifugation studies, a molecular weight in the range of 330,000 is suggested and there may be an additional component of considerably greater size. Partial purification can be achieved by ion exchange chromatography. The purified fraction was completely inactivated by incubation with trypsin. Partially purified fractions inhibit cationic platelet aggregators such as collagen, polylysine, and hexadimethrine but do not affect anionic aggregators such as succinylated collagen and sodium stearate. Normal plasma and serum inhibit succinylated collagen and stearate. Stearate is inhibited by crystalline albumin and Cohn fraction IV-4. It is suggested that plasma proteins may regulate platelet adhesion to collagen and other vessel wall materials.