Regulation by membrane sialic acid of β1H-dependent decay-dissociation of amplification C3 convertase of the alternative complement pathway

Abstract

Sheep erythrocytes in their native state did not activate the alternative complement [C] pathway, as measured by lysis in dilutions of normal human serum containing [ethylenebis(oxyethylenenitrilo)] tetraacetic acid but acquired this capacity after membrane sialic acid residues were removed (by sialidase) or modified (by NaIO4). Activation of the alternative pathway by sheep erythrocytes required removal or modification of at least 40% of the membrane sialic acid to reach threshold, and it increased proportionately when larger amounts of sialic acid were affected. Studies with isolated proteins of the alternative pathway demonstrated that the altered erythrocyte membranes resembled natural activators in protecting bound C3b from inactivation by C3b inactivator and .beta.1H and protecting bound amplification C3 convertase (C3b,Bb) from decay-dissociation by .beta.1H. A 1% decrease in intact sialic acid was associated with a 1% decrease in .beta.1H activity in decay-dissociation of membrane bound C3b,Bb. Because removal of the C8 and C9 C atoms from the polyhydroxylated side chain of sialic acid by oxidation with NaIO4 was functionally equivalent to removal of the entire sialic acid moiety, secondary effects of the latter reaction, such as diminution of the negative charge of the membrane or exposure of penultimate galactose residues, were not considered to be responsible for the altered activity of .beta.1H. Facilitation by membrane sialic acid residues of the interaction between bound C3b and .beta.1H may be essential to prevent the particle from effectively activating the alternative pathway.