Development of methionine synthase, cystathionine-beta-synthase and S-adenosyl-homocysteine hydrolase during gestation in rats.

The developmental onset of three homocysteine metabolizing enzymes in the rat conceptus w as investigated. Cystathionine-s- synthase and methionine synthase w ere assayed from day 10 to day 20 of gestation in decidual and placental tissue, from day 10 to day 12 of gestation in embryonic tissue, from day 14 to day 20 of gestation in fetal liver and from day 14 to day 20 of gestation in fetal tissue without liver. O n each day, material w as obtained from at least four conceptuses from two dams. S-adenosylhomocy steine hydrolase was assayed in neurulating conceptuses in decidual tissue, parietal yolksac plus ectoplacental cone, visceral yolksac plus amnion and embryo proper. Conceptuses w ere pooled from seven (day 9.5 of gestation) or three (days 10,5 an d 11.5 of gestation) dams. In em bryonic and fetal tissue cystathionine-s-synthase first occurred in fetal liver. D uring the organogenic phase it was present only in decidual tissue. Methionine synthase was present in all tissues from the first gestational day investigated and S-adenosylhom ocysteine hydrolase was present in all tissues throughout the neurulating period. O u r results indicate that the hom ocysteine— methionine cycle, which is crucial to transmethylation reactions, is functional during the neurulating period in embryonic tissue. O w ing to the absence of cystathionines-synthase at this stage of development in embryonic tissue, the hom ocysteinyl m oiety is conserved in the homocysteine-met hionine cycle.