Synthesis and secretion of γ-globulin by lymph node cells VII. The cell-free incorporation of galactose and sialic acid into the carbohydrate component of endogenous and exogenous immunoglobulin

Abstract

The incorporation of galactose and sialic acid into endogenous and exogenous rabbit immunoglobulin by glycosyltransferases associated with the microsomal fraction of lymph node cells is described. The endogenous product was firmly associated with the microsomes and could be solubilized by treatment with deoxycholate. In contrast, the exogenously added asialo-immunoglobulin G labeled with sialic acid was found in the soluble fraction in the absence of deoxycholate treatment. When the two glycopeptide fractions of immunoglobulin G, obtained by pepsin digestion and gel filtration, were examined for radioactivity, differences between the labeled endogenous and exogenous products were observed. Endogenous immunoglobulin, labeled with either galactose or sialic acid, showed an elution pattern similar to that of the glycopeptide fractions of authentic immunoglobulin G. When exogenously added asialo-immunoglobulin G labeled with sialic acid was examined, more than 95% of the radioactive material was found in only one of the two glycopeptide fractions. Similarly, examination of the glycopeptides obtained from exogenous agalacto-immunoglobulin G labeled with galactose revealed incorporation into this same fraction. However, most of the galactose-labeled material was found in a peptide elution position not consistent with that of authentic immunoglobulin G. Finally, while the incorporation of sialic acid into endogenous immunoglobulin G was not affected by the addition of other asialo-proteins, the presence of such asialo-protein acceptors inhibitedd the incorporation of sialic acid into exogenously added asialo-immunoglobulin G.