Search

The biosynthesis of lutropin (luteinizing hormone, LH) containing abnormally processed oligosaccharides has been studied in rat gonadotroph-enriched primary culture in the presence of swainsonine, an inhibitor of the N-linked oligosaccharide processing in the Golgi complex. The inhibitor led to time- and dose-dependent accumulation of highly alkaline LH molecules (pI 9.6, 10.0) with high binding affinity for concanavalin A (ConA), and concomitant decreases of less alkaline components (pI 8.0, 8.5, 8.8) with less affinity for the lectin. The accumulation of LH molecules was much amplified after gonadotrophin-releasing hormone (GnRH) stimulation. The characterization of the oligosaccharide structure performed by means of enzyme treatment and serial chromatography on immobilized ConA, Ricinin communis agglutinin-120 (RCA-120), and wheat germ agglutinin (WGA) showed that swainsonine-induced LH contained principally the hybrid-type oligosaccharide, which was processed by the addition of terminal sialic acid residues on one branch of each N-linked oligosaccharide in both subunits. Small amounts of LH molecules containing asialo or partially sialylated hybrid oligosaccharides were also synthesized. The data for in vitro steroidogenic activities of swainsonine-induced LH indicated that terminal sialic acid on one branch of the hybrid-type oligosaccharides decreased the full agonist activity by approximately 50%.