RESPONSE OF THE METHIONINE SYNTHASE SYSTEM TO SHORT-TERM CULTURE WITHHOMOCYSTEINE AND NITROUS-OXIDE AND ITS RELATION TO METHIONINE DEPENDENCE

We compared the metabolic response of a methionine(Met)-dependent (P60 ) human glioma cell line with that of a Met-independent variant (P60H) when cultured in a homocysteine (Hcy) medium and exposed to N2O. In H cy medium (without Met), remethylation of Hey in P60H cells was enhanc ed and supported growth, whereas remethylation was low in P60 cells, w hich failed to thrive under these conditions. Both cell types seemed t o contain adequate amounts of folates and total cobalamin (Cbl). P60 c ells showed increased total and methylcobalamin (CH(3)Cbl) content aft er the shift to a Hcy medium, but the high, stable level of CH(3)Cbl d etected in P60H cells was not attained, Further metabolic differences were induced by N2O exposure, which markedly reduced Met-synthase acti vity in cell-free extracts in both cell lines and completely blocked i ntact-cell Hcy remethylation in P60, whereas Hcy remethylation was onl y partly inhibited in P60H cells cultured in Met medium. The residual Hcy remethylation in P60H cells may be related to only a moderate depl etion of CH(3)Cbl. The resulting high CH(3)Cbl level relative to Met-s ynthase activity during N2O exposure was even higher in Hcy medium. Th ese findings in P60H cells probably reflect increased provision of Cbl to support Hey remethylation under metabolic strain. The inability of P60 to furnish CH(3)Cbl to the enzyme may explain both the Met-depend ent phenotype and the increased sensitivity of Hcy remethylation to N2 O exposure in these cells. (C) 1997 Wiley-Liss, Inc.