Influence of α- and β-Subunits on the Kinetics of Formation and Activity of Native and Hybrid Molecules of LH and Human Chorionic Gonadotropin1

Abstract

A radioreceptor assay for LH (Endocrinology91: 901, 1972) was utilized to study the kinetics of formation of native and hybrid molecules of bovine LH (bLH) and human chorionic gonadotropin (hCG). Conditions for combination of subunits were as described in (J Biol Chem244: 5110, 1969). The formation of hCG and bLH from their subunits was best represented by a rate equation 1st order in each of the subunits (overall 2nd order). The formation of various hybrid molecules, as between hCG-α/bLH-β and bLH-α/hCG-β, also followed 2nd order kinetics. Rates of formation were always greater when the a-subunit of hCG was involved. For example, the rates of formation of the hCG-α/ bLH-β and hCG-α/hCG-β molecules were similar to each other but significantly greater than the rates of formation of bLH-α/bLH-β or bLH-α/ hCG-β molecules. The radioligand uptake inhibitory activity of various molecules studied seemed most strongly influenced by the nature of the β-subunit. The activities of hCG-β/hCG-α or hCG-β/bLH-α molecules were considerably greater than those found for bLH-β/bLH-α or bLH-β/hCGa. The activity of bLH-β/hCG-α hybrid was similar to that of native bLH, but less than that of native hCG. The inhibitory activity of hCG-β/bLH-α was similar to that of native hCG and greater than that of native bLH. The slope of the uptake inhibition curve of the molecules formed from association of subunits was found to be influenced by the nature of the β-subunit. The slope for hCG was greater than the slope for bLH. Hybrids formed with the β-subunit of bLH had slopes similar to that of intact bLH. Hybrids formed with the β-subunit of hCG had slopes similar to that of intact hCG. Similar results were obtained when various combinations of subunits of hCG and porcine LH were studied. In this system, therefore, the provenance of the P-subunit appears to play the predominant role in determining the affinity of the newly formed molecule for its receptor. Such results are in agreement with earlier studies on subunits of glycoprotein hormones which demonstrate that the qualitative biological activity of any α-β dimer is determined by the nature of the β-subunit. (Endocrinology93: 938, 1973)