Hormonal and immunological aspects of the phylogeny of sex steroid binding plasma protein.

Sex steoid binding plasma protein (Sbp) in man and in monkeys binds the androgens dihydrotestosterone and testosterone and the estrogen estradiol with high affinity (Kd approximately 0.5, 1, and 2 nM, respectively). Detailed studies of steroid binding specificity give the same results in all primates, except that in humans and chimpanzees estrone does not compete for dihydrotestosterone binding. In other mammals, Sbps of Artiodactyla and Lagomorpha have the same range of affinities for androgens but they do not bind estradiol to any significant extent (Kd > 280 nM). The dog has an unusual Sbp (Kd for dihydrotestosterone, 7.1 nM; for estradiol, 125 nM), and rodents do not have a specific dihydrotestosterone-binding plasma protein. Gel filtration and immunoelectrophoretic experiments have been performed with a monospecific antiserum against human Sbp. The results indicate variable crossreactivities with Sbps of primates (from complete in chimpanzee and gorilla to weak in Prosimii). No crossreaction was observed with specific androgen-binding plasma proteins of other species. These results suggest the evolutionary emergence of bifunctional Sbp.