Subunit-specific sulphation of oligosaccharides relating to charge-heterogeneity in porcine lutrophin isoforms.

Abstract

Lutrophin (LH) consists of an array of isoforms with different charges and bioactivities. This study was undertaken to clarify specifically how oligosaccharides of alpha and beta subunits contribute to LH isoform charges. Porcine LH (pLH) was separated into four isoforms by isoelectric focusing (IEF), followed by subunit isolation. Their oligosaccharides were released by hydrazinolysis, labelled by reduction with NaB3H4, and fractionated by HPLC with a Mono Q column into five populations differing in the number of sulphate (S) and sialic acid (N) residues, designated as Neutral, N-1, S-1, S-N and S-2. Oligosaccharides were predominantly sulphated (S-1 and S-2) and infrequently sialylated (N-1 and S-N). Further analysis, including concanavalin A (Con A) affinity chromatography, desialylation, desulphation, sequential exoglycosidase digestion and methylation, clarified the structures of the acidic oligosaccharides. All were of the biantennary complex type. Their two peripheral branches were SO4-4GalNAc beta 1-4Glc-NAc and GalNAc beta 1-4GlcNAc or GlcNAc in S-1, SO4-4GalNAc beta 1-4GlcNAc and Sia alpha 2-6Gal beta 1-4GlcNAc in S-N, and (SO4-4GalNAc beta 1-4GlcNAc)2 in S-2 (where GalNAc is N-acetylgalactosamine and GlcNAc is N-acetylglucosamine). Ten percent of S-1 and of S-N had a bisecting GlcNAc residue. Sulphate residues occurred in nearly the same amount for both subunits; however, the alpha and beta subunits were sulphated differently. S-1 predominated in the alpha subunit, while S-1 and S-2 were major components in the beta subunit.(ABSTRACT TRUNCATED AT 250 WORDS)