Amino acid-conferred protection against melphalan—Characterization of melphalan transport and correlation of uptake with cytotoxicity in cultured L1210 murine leukemia cells

The uptake of melphalan by L1210 cells was reduced to one-sixth of controls by physiological concentrations of the l-isomers of leucine and glutamine, and this decrease was accompanied by a corresponding decrease in cytotoxicity. Cytotoxicity was estimated by treatment of cells with melphalan for 35 min in phosphate-buffered saline containing bovine serum albumin and glucose followed by clonal growth of the surviving cells in soft agar. It was prominent within a critical region of net melphalan uptake of 2–5 pmoles/105 cells. Inhibition analysis revealed that at cytotoxic concentrations melphalan is transported by a high-affinity amino acid transport system of the leucine (l) type. The values of the Michaelis constants (Km) for l-leucine, a protective amino acid, l-valine, a minimally protective amino acid, and melphalan were 6, 58 and 19 μM respectively. These results suggest that the ability of amino acids to protect L1210 cells from melphalan cytotoxicity is related to their affinities for the leucine carrier sites.