Response of the methionine synthase system to short-term culture with homocysteine and nitrous oxide and its relation to methionine dependence
✍ Scribed by Torunn Fiskerstrand; Per M. Ueland; Helga Refsum
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- French
- Weight
- 107 KB
- Volume
- 72
- Category
- Article
- ISSN
- 0020-7136
No coin nor oath required. For personal study only.
✦ Synopsis
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 N 2 O. In Hcy medium (without Met), remethylation of Hcy in P60H cells was enhanced and supported growth, whereas remethylation was low in P60 cells, which failed to thrive under these conditions. Both cell types seemed to contain adequate amounts of folates and total cobalamin (Cbl). P60 cells showed increased total and methylcobalamin (CH 3 Cbl) content after the shift to a Hcy medium, but the high, stable level of CH 3 Cbl detected in P60H cells was not attained. Further metabolic differences were induced by N 2 O exposure, which markedly reduced Met-synthase activity in cell-free extracts in both cell lines and completely blocked intact-cell Hcy remethylation in P60, whereas Hcy remethylation was only partly inhibited in P60H cells cultured in Met medium. The residual Hcy remethylation in P60H cells may be related to only a moderate depletion of CH 3 Cbl. The resulting high CH 3 Cbl level relative to Met-synthase activity during N 2 O exposure was even higher in Hcy medium. These findings in P60H cells probably reflect increased provision of Cbl to support Hcy remethylation under metabolic strain. The inability of P60 to furnish CH 3 Cbl to the enzyme may explain both the Met-dependent phenotype and the increased sensitivity of Hcy remethylation to N 2 O exposure in these cells.