Binding of alpha(2)-macroglobulin and limulin: regulation of the plasma haemolytic system of the American horseshoe crab, Limulus

The mediator of haemolysis in the plasma of the horseshoe crab, Limulus pol yphemus, is limulin, a sialic acid-binding lectin. The haemolytic activity of limulin is inhibited by thiol eater-reacted forms of Limulus alpha(2)-ma croglobulin, the third-most abundant protein of the plasma. Limulus alpha(2 )-macroglobulin that has experienced cleavage of its internal thiol ester b ond, consequent to reaction with proteases, or with the small primary amine , methylamine, reduces the haemolytic activity of limulin when present at m olar excesses that approximate the relative concentrations of these two pro teins in the plasma. Native, unreacted Limulus alpha(2)-macroglobulin has n o effect on the haemolytic activity of limulin. Limulin binds thiol ester-r eacted forms of Limulus alpha(2)-macroglobulin both in a solid-phase assay and in solution with an avidity 10-25 times higher than native, unreacted L imulus alpha(2)-macroglobulin. Protease-reacted alpha(2)-macroglobulin func tions as a marker for the presence of foreign proteases in the blood of Lim ulus, and thus of pathogenic organisms that release proteases as facilitato rs of invasion and pathogenicity. Modulation of the haemolytic system repre sents a novel function for alpha(2)-macroglobulin.