Effects of Removal of Carboxy-Terminal Extension from Equine Luteinizing Hormone (LH) β-Subunit on LH and Follicle-Stimulating Hormone Receptor-Binding Activities and LH Steroidogenic Activity in Rat Testicular Leydig Cells*

Residues 121–149 of equine LHβ (eLHβ) were removed by a simple mild acid treatment procedure. The modified eLHβ, des(121–149)eLHβ, was isolated by gel permeation chromatography on Sephacryl S-200. Recombination of des(121–149)eLHβ with eLHα and ovine LHα (oLHα) produced LH derivatives as efficiently as recombination with native eLHβ. In rat testicular LH radioligand assay systems employed in this study the potencies of the resulting LH preparations were, in order of decreasing potency: des(121–149)eLHβ:eLHα hybrid > eLH > eLHα + β > oLH > des(121–149)eLHβ:oLHα > oLHα + eLHβ (1:0.82:0.67:0.15:0.02:0.006, eLH tracer; 1:0.88:0.67:0.21:0.02:0.006, hCG tracer). In a horse testicular LH radioligand assay with eLH tracer, only the equine LH derivatives were active, and the order of potencies was the same: des(121–149)eLHβ:eLHα hybrid > eLH > eLHα + β (1:0.58:0.46). In a rat testicular Leydig cell steroidogenesis assay, eLH was the most active preparation, but the relative potencies of the other preparations rema...