Studies on the immunochemical relatedness among tetrapod gonadotropins and their subunits with antisera to sea turtle hormones

Immunological relatedness among gonadotropins from the four classes of tetrapods was studied with three antisera ($\text{a}\text{s}$) raised in rabbits against follicle-stimulating hormone (FSH), luteinizing hormone (LH), and LHβ subunit from the green sea turtle, Chelonia mydas. Only pituitaries or FSH from turtles showed appreciable cross-reaction in the homologous RIA for Chelonia FSH, and preparations from the heterologous suborder Pleurodira showed weaker and less complete cross-reactivity than from species of Cryptodira. However, the Chelonia FSH $\text{a}\text{s}$ was capable of neutralizing the biological activity of crocodilian FSH and showed specific binding to ¹²⁵I-labeled FSH from birds and mammals (but not amphibians). Considerable immunorelatedness was evident among the LH preparations of different species and this appeared to result primarily from common antigenic determinants on the α subunits of LH. First, all chelonian, crocodilian, and avian LH preparations showed high activity in RIA based on anti-Chelonia LH serum and ¹²⁵I-labeled Chelonia LHα as tracer; in fact, several of these heterologous species of LH were more potent than the Chelonia LH. LH from eutherian mammals (human, ovine, bovine) showed essentially no cross-reactivity, but preparations of ovine LHα and kangaroo LH showed measurable activity in LHα RIA. Second, the antiserum for LH, which contained antibodies to the LHα subunit, neutralized the biological activity of LH from other species of turtles, crocodilians, and a mammal (sheep). Third, ¹²⁵I-labeled preparations of avian (turkey) and mammalian (ovine and rat) LH showed specific binding to this antiserum. In contrast, parallel tests with an antiserum raised against Chelonia LHβ showed high cross-reactions only with LHs from other species of turtles; i.e., little or no cross-reaction with any nonchelonian species was indicated in RIA, neutralization tests, or binding studies with ¹²⁵I-labeled hormones. Gonadotropins from amphibians and snakes showed no cross-reaction in any tests with any of the three antisera used. This conspicuous lack of immunological cross-reactivity of snake gonadotropins, especially with antibodies to LHα, provides further evidence for a pronounced structural divergence between the hormones of squamates and the other reptilian orders.