Binding of iodinated rat and ovine prolactins to prolactin receptors and to its antibodies

Abstract Most studies of prolactin receptors in rat tissues have not used the homologous 125 I-labeled rat prolactin as tracers, but rather 125 I-labeled ovine or human prolactin. We have compared the effect of different methods of iodination on the specific binding of rat and ovine prolactin to sites in the seminal vesicle of rats and the liver of mice post-partum. Ovine prolactin, either iodinated with lactoperoxidase or with mild chloramine-T (10 μg), showed 3 times the specific binding of correspondingly-iodinated rat prolactin. This greater sensitivity of rat prolactin to oxidative damage during iodination, as compared with ovine prolactin, is further shown by the difference in Sephadex G-100 elution constant of unlabeled and labeled rat prolactin. This difference was absent in the case of ovine prolactin. Parallel studies of the binding of the labeled hormone to homologous antibody revealed that immunoreactivity of labeled ovine prolactin was not affected by any of the iodination methods. Rat-prolactin immunoreactivity was depressed by lactoperoxidase iodination as compared with chloramine-T iodination. Rat prolactin was also less potent than ovine prolactin in inhibiting the binding of homologous and heterologous labeled hormone to its receptors, to a larger degree than could be expected from the bioassay potency of the various hormone preparations. These results reflect the greater sensitivity to damage of the biologically active site of rat prolactin, as compared with the ovine hormone.