Plasma cortisol transport and primate evolution

Primates have diverged into three major evolutionary groups: prosimians, Old World primates, and New World primates; the last group is distinguished by high circulating cortisol concentrations and resistance to the action of glucocorticoids. We have studied a large spectrum of primate species within these groups to characterize the phylogenetic relationships of cortisol-binding globulin (CBG) among them. The CBG in each species was found to be glycosylated, as judged from lectin interactions, and to exhibit an electrophoretic mobility similar to that of human CBG. Although the CBG affinity for cortisol differed among species, the effects of changes in temperature on the CBG affinity were similar. Strikingly, the CBG-binding capacity of plasma in the New World primates was 1/10th to 1/100th those in the Old World primates and prosimians, while the CBG-binding affinity for cortisol was lower. The reduced capacity and affinity of CBG result in a markedly higher fraction of unbound plasma cortisol in the New World primates than in the Old World primates or the prosimian species examined. This evolutionary pattern of CBG may be a compensatory mechanism for the target organ resistance to glucocorticoids that characterizes the New World monkeys.