Biotransformations of pituitary luteinizing hormone in serum and urine. II. Evidence for reduced potency following urinary excretion.

The renal clearance of tritiated and methylated ovine pituitary luteinizing hormone (LH) has been determined in mature male rats, and the urinary product has been characterized with regard to molecular size, charge, and heterogeneity. [3H]Methylated-LH was found to have a renal clearance of 0.17+/-0.03 ml/min/100 g body weight, compared to a value of 0.99+/-0.18 for tritiated inulin. This indicates that LH is not secreted and that much of the filtered hormone is reabsorbed from the tubular lumen. Evidence is presented which shows that urinary ovine [3H]methylated-LH is not extensively degraded; yet, the biological potency is only 10-20% that of pituitary and serum LH. The large reduction in potency is a result of excretion since the in vitro incubation of LH with urine does not produce the same effect. The biotransformation occurring during excretion results in charge heterogeneity of LH; this may arise from either inhibitor binding or enzymic alterations.