Glycolysis In several tumor cell lines grown in tissue culture was inhibited by methionine. Kfrsten murine sarcoma virus-transformed rat kidney cells (K-NRK) were inhibited 60-75% by 10 mM methionine, whereas normal rat kidney (NRK-49F) cells showed little or no inhibition. Inhibition of glycolysis in K-NRK cells was manifest 2-4 hr after exposure to the amino acid. Glycolysis in a chemically transformed cell line of Madin-Darby canine kidney cells was also sensitive to methionine, but maximal inhibition (75%) required 18-24 hr of incubation with the amino acid. Under the same conditions glycolysis in the nontransformed canine cells was less than 20% inhibited by methionine. In Ehrlich ascites tumor cells grown in tissue culture, 10 mM methionine inhibited glycolysis by about 50%. Inhibition of glycolysis, even by 50 mM methionine, was rapidly reversible. Within 2 hr after removal of methionine the rate of glycolytic activity was restored to that observed in control cells. Furthermore, inhibition by methionine required a minimum level (7%) of serum in the growth medium and inhibition was not sensitive to cycloheximide. Only amino acids that are transported by system A (including the nonmetabolized analogue methylaminoisobutyric acid) specifically inhibited glycolysis in tumor cells. The only exception was phenylalanine, which was toxic to both transformed and normal cell lines.It has been known for many years that tumors have an altered methionine metabolism (1)(2)(3)(4)(5). The most striking differences are (i) the inability of many tumor cell lines to grow at low concentrations of methionine or when methionine was replaced by homocysteine, and (ii) a low level of intracellular methionine and S-adenosylmethionine. Yet these same cell lines synthesize methionine and incorporate it into proteins at rates comparable to those in normal cells. Furthermore, most transformed cell lines take up methylaminoisobutyrate (MeAIB), methionine, and other substrates via system A for amino acid transport at a considerably faster rate than their "normal" parent cells (6-8). These puzzling and paradoxical observations are unexplained. Preliminary experiments (unpublished data) showed that MeAIB, a specific substrate for system A (9), inhibited the growth of the MDCK-T1 cell line, a chemically transformed variant of Madin-Darby canine kidney (MDCK) cells (10), but did not significantly inhibit the growth of MDCK cells. A similar sensitivity to MeAIB or to methionine was observed with Ehrlich ascites tumor (EAT) cells grown in tissue culture (unpublished data of P.B. and D. Westcott). We also observed that glycolysis of confluent normal rat kidney cells (NRK-49F) was stimulated by transforming growth factor (3 6) and that the stimulation of glycolysis by TGF-,3 was inhibited in the presence of high concentrations of methionine (11). These observations prompted us to explore the effect of methionine on glycolysis of transformed cells grown in tissue culture and to ask if culture conditions influenced the response of these ce...