PREVENTION OF NEURAL-TUBE DEFECTS BY AND TOXICITY OF L-HOMOCYSTEINE IN CULTURED POSTIMPLANTATION RAT EMBRYOS

Mild hyperhomocysteinemia is frequently observed in mothers who gave b irth to a child with a neural tube defect (NTD). In a previous study w e showed L-homocysteine was embryotoxic to gestational day 10 (GD10) r at embryos in culture, however, no NTDs were observed. We therefore in vestigated the effect of L-homocysteine on the development of neural p late stage (GD9.5) rat embryos. Other objectives of this study were in vestigation into whether the embryotoxicity of L-homocysteine could be attenuated by compounds related to its metabolism and clarification o f the mechanism of L-homocysteine embryotoxicity. In GD9.5 rat embryos L-homocysteine was not toxic at 1- and 2-mM concentrations. Rather at these concentrations it promoted development of the rat embryos in se rum that without supplementation caused NTDs in the embryos. L-Methion ine had the same preventive effect at even lower concentrations, but f olinic acid (1 mM) did not improve embryonic development. N-5-Methylte trahydrofolate (5-CH3-THF) (100 mu M), L-serine (6 mM), and L-methioni ne (6 and 12 mM) attenuated the embryotoxicity of L-homocysteine (6 mM ) in GD10 rat embryos. Vitamin B-12 (10 mu M) completely abolished the embryotoxicity of L-homocysteine, which was shown to be mediated by c atalysis of the spontaneous oxidation of L-homocysteine to the less to xic L-homocystine. In GD11 rat embryos, both L- and D-homocysteine wer e readily taken up when added to the culture (3 mM) and increased embr yonic S-adenosylhomocysteine (SAH) levels 14- and 3-fold, respectively . This difference was shown to be caused by the stereospecific prefere nce of SAH hydrolase. We propose the basis for L-homocysteine embryoto xicity is an inhibition of transmethylation reactions by increased emb ryonic SAH levels. (C) 1994 Wiley-Liss, Inc.