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.