The third component of complement: Covalent attachment of a radioactive sugar to the labile binding site of C3 via the alternative pathway

J. Mann, R. O'Brien, M. K. Hostetter, C. A. Alper, F. S. Rosen, B. M. Babior

Research output: Contribution to journalArticlepeer-review

Abstract

A complement- (C) fixing particle consisting of agarose beads to which 5-thioglucose was attached by a -S-S-bond (agarose-thioglucose) was employed to investigate the mechanism of attachment of C3 to surfaces. When whole serum containing [ 125I] C3 was incubated with agarose-thioglucose, labeled C3b was taken up in a form that was not removed by 2 M NaCl but was released by 10 mM dithiothreitol. Deposition of DDT-releasable C3b was dependent upon the alternative pathway of C activation. Gel electrophoresis of DTT-releasable C3b from similar experiments was dependent upon the alternative pathway of C activation. Gel electrophoresis of DTT-releasable C3b from similar experiments performed with unlabeled serum and agarose-[ 3H]thioglucose showed that the liberated C3b contained a molecule of radioactive thioglucose attached to the α'-chain by a covalent bond that was stable to mercaptoethanol. We propose that the thioglucose-α' chain bond was formed during the course of C activation by a reaction between the 'labile binding site' of newly released C3b and the (then) particle-bound sugar. This formulation implies that the reaction by which C3b attaches to 5-thioglucose in this system is the reaction responsible for opsonization by C3b, and that the C3b-linked sugar represents a marker for the labile binding site. Incubation of the particle-bound C3b in serum resulted in the cleavage of the covalently linked α'-chain to several smaller polypeptides, the major cleavage product having a m.w. of 70,000.

Original languageEnglish (US)
Pages (from-to)2370-2372
Number of pages3
JournalJournal of Immunology
Volume126
Issue number6
StatePublished - 1981
Externally publishedYes

ASJC Scopus subject areas

  • Immunology

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