The first component of human complement: on the mechanism of activation by some carbohydrates.

Four synthetic saccharides (mono-, di-, tri-, tetra-) linked to the carrier 8-methoxycarbonyloctanol were used in the presence or absence of a polysaccharide (PS) purified from ant venom to study the activation of C1 and the consumption of C4 in normal serum. The carrier was essential for the recorded activities of the saccharides. The alpha D mannose-O-carrier did not cause C1 activation and C4 consumption by itself, but it enhanced these activities when combined with the venom PS. The alpha L fuc(1 leads to 4)beta D glc-NAc-O-carrier caused C1 activation but no C4 consumption by itself at a high concentration (1.4 mg/ml), and enhanced C1 activation and C4 consumption in combination with the venom PS. The beta D gal(1 leads to 3)alpha L fuc(1 leads to 4)beta D glc-NAc-O-carrier caused both C1 activation and C4 consumption in normal human serum by itself. The alpha L fuc(1 leads to 2)beta D gal[alpha L fuc(1 leads to 4)]beta D glc-NAc-O-carrier neither caused C1 activation and C4 consumption by itself, nor did it enhance these activities when combined with the venom PS. Neither EAC1hu nor EAC1hu bound 3H-PS or 3H-beta D gal(1 leads to 3)alpha L fuc(1 leads to 4)beta D glc-NAc-O-carrier. Also, the precursor human C1 on EA (EAC1) was not activated by the venom PS. Thus, the overall evidence shows that human precursor C1 is activated by these carbohydrates solely by an interaction with C1q subunit of macromolecular C1.