High in vivo rates of methionine biosynthesis in transformed human and malignant rat cells auxotrophic for methionine

Unlike normal cells, malignant rat and two simian virus 40 transformed human cell lines can neither grow nor survive in B₁₂ and folate supplemented media in which methionine is replaced by homocysteine. Yet three lines of evidence indicate that the malignant and transformed cells synthesize large amounts of methionine endogenously through the reaction catalyzed by 5 methyltetrahydropteroyl L glutamate: L homocysteine S methyltransferase (EC 2.1.1.13). (1) The activities of this methyltransferase were comparable in extracts of malignant and normal cells. (2) The uptake of radioactive label from [5 ¹⁴C] methyltetrahydropteroyl L glutamic acid (5 Me H₄PteGlu) was at least as great in the malignant cells as in the normals and was nearly totally dependent on the addition of homocysteine, the methyl acceptor; furthermore, 59-84% of the label incorporated by cells was recovered as methionine. (3) The malignant and transformed cells were unable to grow in homocysteine alone, while in the presence of otherwise limiting amounts of exogenous methionine, homocysteine greatly stimulated the growth of these cells. The minimum concentration of methionine necessary to initiate growth of normal and of malignant and transformed cells was the same in all lines, and the maximal growth rates at optimal methionine concentrations did not distinguish the normal from the malignant and transformed cells. The endogenously synthesized methionine was readily incorporated into high molecular weight substances by the malignant and transformed cells, indicating the absence of a generalized defect in utilization. Inoculation at high density did not allow these cells to grow on homocysteine in methionine deficient media nor did the omission of the seven nonessential amino acids affect growth, and suggests that the dependence on exogenous methionine is not due to leakage of methionine alone or with other amino acids. The requirement for exogenous methionine for growth despite high levels of endogenous synthesis, is presently unexplained but appears to distinguish at least certain oncogenically transformed, but otherwise widely varying, cell lines from normals.