Characteristics of Hybrids of Ovine LH and Human Glycoprotein Hormone Subunits in Rat and Chicken in vitro Test Systems

Hybrids of ovine luteinizing hormone (oLH) and human chorionic gonadotropin (hCG) subunits were prepared and characterized. Human luteinizing hormone (hLH), hCG, oLH, and the hybrid gonadotropins hCGα:oLHβ and oLHα:hCGβ were tested for biological potency using three assays: 1) rat testis radioreceptor assay, 2) rat Leydig cell steroidogenesis assay, and 3) rooster testis steroidogenesis assay. In each assay the hybrid gonadotropins behaved characteristically as the parent gonadotropins from which the beta subunits were obtained. In the rat testis radioreceptor assay, hCG and oLHα:hCGβ were about 3 times as potent as oLH and hCGα:oLHβ. When assayed using the rat Leydig cell steroidogenesis assay, hCG and oLHα:hCGβ were about 1.25 times as potent as oLH and hCGα:oLHβ. In the rooster testis steroidogenesis assay, oLH and hCGα: oLHβ were about 300 times more potent than hCG or oLHα:hCGβ. The hLH behaved similarly but not identical to hCG in each assay system. This study shows that the structure(s) responsible for differences in potency between human and ovine LH resides in the beta subunit. The relatively similar behavior of hLH and hCG in these assays as contrasted to oLH indicates that the structure responsible for differing LH potencies is not in the carboxyl terminal 26 amino acids which are unique to chorionic gonadotropins. The high sialic acid content of hCG also appears unrelated since 80% desialated hCG was slightly less potent than hCG in the chicken test system. The conclusion is that one or more point mutations have occurred in the evolution of the LH beta subunit which render human glycoprotein hormones with luteinizing hormone activity almost inactive in the chicken, or more specifically, the chicken receptor system. The glycoprotein hormone alpha subunits from the sheep and human species appear functionally identical.