A subclass of albumin recognized by inhibition of phosphatidylethanolamine-mediated agglutination of mouse erythrocytes

Abstract

Agglutination of mouse erythrocytes by non-choline phospholipids is inhibited by a factor in mammalian sera. The inhibitor cochromatographed with albumin on dye-agarose conjugates, was retained by an anti-albumin affinity column, was neutralized by anti-albumin antibody and found in a serum fraction in which only albumin could be detected. A variety of commercial preparations of albumin (fraction V, crystalline) did not inhibit. However, they acquired potent inhibitory activity when treated with low molecular weight thiols. The inhibitory activity of serum was increased 8-fold by treatment with dithiothreitol. Other proteins were not activated in this way. Inhibitory activity increased with average free sulphydryl content of treated albumin, up to six thiol groups per molecule. Alkylation of these sulphydryl groups did not diminish inhibitory activity. Thiols also induced polymerization of albumin. Inhibitory albumin in serum was largely monomeric. We propose that the inhibitor is a type of serum albumin which is lost or inactivated during preparation of commercial albumin, and which shares a structural feature, necessary for inhibition, with thiol-reduced albumin and the ligand on mouse erythrocytes.