Cystathionine synthase2 catalyzes the formation of cystathionine from homocysteine and serine. This is an essentially irreversible reaction in the transsulfuration pathway through which the sulfur of methionine is trans ferred to the carbon skeleton of serine for the ultimate formation of cysteine (fig. 1). Patients with congenital deficiency of cystathionine synthase [23] ex hibit the clinical features of the well-known disease, homocystinuria [17]. The product of the cystathionine synthase reaction, cystathionine, is found only in the free state and is in particularly high concentration in brain tissue, espe cially the brain of primates [33]. The precise function of cystathionine in brain is not known.

In a limited developmental study, FINKELSTEIN [7] found the activity of cystathionine synthase in liver and brain of newly bom rats to be about one half that in animals recently weaned. He did not, however, delineate the de velopmental pattern in the fetus or in the young animal in the first 2-3 weeks of life. Otherwise informative studies of primate tissues also have not deline ated the developmental changes of the perinatal period. Thus, we found cys tathionine synthase activity in the liver of first trimester monkey fetus to be approximately 70% of the activity in the liver of the mature animal [35], There were no marked changes in enzymatic activity in the brain from the first trimester of gestation to maturity [35]. In a study of aborted human ma-