Contribution of subunits to the function of luteinizing hormone/human chorionic gonadotropin recombinants

The alpha- and beta-subunits of hCG and of ovine and porcine LH were used to prepare all nine homologous and heterologous alpha beta-recombinants. EAch purified recombinant was assayed in vitro, using dispersed Leydig tumor cells derived from the M5480P tumor, for its ability to stimulate steroidogenesis and to inhibit [125I]iodo-hCG binding. It was found that the potency of a given recombinant in both assays was most similar to that of the hormone from which the beta-subunit was derived. For example, hCG and hCG beta-containing recombinants were invariably more potent than LH and LH beta-containing recombinants. However, within groups of recombinants containing a common beta-subunit, the alpha-subunit exhibited modulatory effects on the biological potencies. The different observed potencies did not result from alpha beta dissociation since the recombinants were stable in dilute solution at 37 C for periods greatly exceeding that of the assay conditions. LH and the LH beta-containing recombinants were found to dissociate more readily from the Leydig tumor cell gonadotropin receptor than hCG and hCG beta-containing recombinants. (These experiments were performed under conditions where internalization was minimal.) However, analogous to the potency measurements, the alpha-subunit contributed to the rate of dissociation. For example, in recombinants with a common beta-subunit, hCG alpha conferred the greatest stability to the hormone-receptor interaction. These results emphasize a positive relationship between receptor occupancy and biological potency. Whereas the beta-subunit of these gonadotropins seems to exhibit the predominant influence in determining potency, it is clear that both subunits contribute to biological activity. This could involve direct effects as well as induced conformational changes in the complementary subunit.