Salvage of 5'-deoxy-5'-methylthioadenosine and L-homocysteine into methionine in cells cultured in a methionine-free medium: a study of "methionine-dependence".

The metabolism of the two methionine precursors, L-homocysteine and 5'-deoxy-5'-methylthioadenosine was compared to the ability of these compounds to support cell growth in a Met-free medium, in the following mammalian cell lines: Raji, CCL 39 and BHK cells. These three cell lines metabolized L-homocysteine and 5'-deoxy-5'-methylthioadenosine into methionine, S-adenosyl-L-methionine and proteins. However there was a discrepancy between metabolic and growth studies: Raji cells could grow on L-homocysteine and on 5'-deoxy-5'-methylthioadenosine, BHK cells could grow on L-homocysteine but not on 5'-deoxy-5'-methylthioadenosine, and CCL 39 cells could not grow either on L-homocysteine or on 5'-deoxy-5'-methylthioadenosine. The metabolism of exogenous methionine, and of methionine endogenously synthesized from 5'-deoxy-5'-methylthioadenosine was studied in CCL 39 and Raji cells, incubated with 25 microM [methyl-14c] methionine + 25 microM 5'-deoxy-5'-methylthioadenosine or 25 microM [methyl-14c] 5'-deoxy-5'-methylthioadenosine + 25 microM methionine: there was no difference between the metabolism of exogenous and endogenous methionine in either type of cell. Our results indicate that i) "methionine dependence" initially described for L-homocysteine [Hoffman, R.M. and Erbe, R.W. (1976) Proc. Natl. Acad. Sci. USA 73, 1523], can also be observed with the other precursor of methionine, ie 5'-deoxy-5'-methylthioadenosine; ii) "methionine-dependence" can not be considered as the inability of a cell to grow on methionine endogenously synthesized from a precursor, but depends on the precursor used, and possibly, on a toxic effect of the precursor in the absence of methionine.