Free alpha-like material from bovine pituitaries. Removal of its O-linked oligosaccharide permits combination with lutropin-beta.

Further characterization of the free alpha subunit immunoreactive material, not combined with beta subunit in extracts of bovine pituitaries, shows that the only significant modifications, relative to alpha subunits themselves, are the oligosaccharide O-linked to threonine-43, and heterogeneity of the carboxyl terminus. Removal of the O-linked carbohydrate with a mixture of glycosidases from Streptococcus pneumoniae results in an alpha-like material capable of combining with lutropin beta subunit and, thus, the presence of the oligosaccharide is responsible for the inability of the free alpha-like material to combine with beta subunits. Amino acid compositions of tryptic peptides spanning the entire sequence indicate no change in amino acid sequence of the free alpha-like material as compared to lutropin alpha. Further, based on the similar behavior reverse phase high performance liquid chromatography of the tryptic peptides as compared to their lutropin alpha counterparts, it is concluded that no additional post-translational modifications are present. The N-linked oligosaccharides of the free alpha-like material most likely contain terminal O-sulfated N-acetylhexosamines (as do the asparagine-linked carbohydrates from the pituitary hormones) as indicated by the presence of 3 mol of sulfate/mol of free alpha-like material and the resistance of these oligosaccharides to enzymatic deglycosylation. The O-linked oligosaccharide does not contain sulfated residues.