Differential secretion of O-glycosylated gonadotropin alpha-subunit and luteinizing hormone (LH) in the presence of LH-releasing hormone.

The pituitary hormones LH, FSH, and TSH are secreted as dimers of two subunits, alpha and beta. The alpha-subunit, identical in all three hormones, is produced by the pituitary in excess of beta and secreted as free subunit. Bovine free alpha does not combine with purified beta-subunit and contains an extra O-linked oligosaccharide not found on dimer alpha. We have developed an assay to quantitate this modified form of alpha in the medium of incubated steer pituitary slices. The assay, based on reverse phase HPLC analysis of radiolabeled alpha-subunit tryptic peptides, shows that under basal conditions, 75% of secreted free alpha is O-glycosylated. When the secretagogue LHRH is added to the slices, a 14-fold increase in LH dimer release is observed, but secretion of the modified alpha is increased by only 2-fold. Our results indicate that the majority of free alpha-subunit secreted by the pituitary contains O-linked oligosaccharide, and that secretion of this form of alpha differs from that of LH dimer.