Combination of rat lutropin subunits occurs early in the secretory pathway.

The combination of lutropin (LH; luteinizing hormone) alpha and beta subunits was examined in rat pituitaries incubated with [35S]methionine or [35S]sulfate. Combination was assessed by using antiserum directed against the beta subunit. The data show that combination of most of the subunits proceeds rapidly, well before the addition of sulfate and prior to the processing of asparagine-linked oligosaccharides to the complex form. Thus, combination appears to initiate in the endoplasmic reticulum and does not require those post-translational modifications. We observed that two forms of the LH-alpha subunit were processed--one that is secreted into the medium not associated with the LH beta subunit and another secreted as part of the alpha-beta dimer. Both forms of the alpha subunit are sulfated, and the data suggest that subsequent to sulfate addition, secretion of free alpha subunit and the dimer occur independently by separate pathways.