Differential Secretion ofO-Glycosylated Gonadotropinα-Subunit and Luteinizing Hormone (LH) in the Presence of LH-Releasing Hormone

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Abstract
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.

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