COMPARISON OF BIOLOGICAL ASPECTS AMONG OVINE LUTEINIZING-HORMONE ISOFORMS WITH CHARGE HETEROGENEITY

Ovine luteinizing hormone (oLH), a pituitary hormone with sulfated asp aragine-linked oligosaccharides, was examined with regard to how its i soforms having different isoelectric points (pIs) modulate in vivo bio logical activity. oLH was separated into five fractions by means of an isoelectric focusing (IEF) column, i.e., IEF fr 1 (pI > 10.78), IEF f r 2 (pI 10.78-10.26), IEF fr 3 (pI 10.22-10.07), IEF fr 4 (pI 9.99-9.8 4), and IEF fr 5 (pI 9.76-9.48). Of these, we studied the three major fractions, IEF fr 2, 3 and 4. The binding of I-125-oLH to rat testis h omogenates was inhibited most strongly by IEF fr 4, followed by 2 and 3. The ability to stimulate cyclic AMP release from dispersed rat Leyd ig cells followed the same sequence. After bolus injections of IEF fra ctions, the plasma immunoreactive oLH levels of male rabbits were meas ured. IEF fr 4 and 3 had longer plasma half-lives than 2. As expected, plasma testosterone levels after injections increased with the additi ve effects of in vitro activity and the rate of clearance from the cir culation, i.e., IEF fr 4 > 3 = 2. In vivo renotropic activity, H-3-thy midine incorporation into the renal DNA of hypophysectomized, castrate d rats, was found only in IEF fr 4. Our study indicates that an oLH is oform with a lower alkali pI had stronger LH receptor binding and in v itro bioactivity and a longer plasma half-life, resulting in stronger in vivo bioactivity.