Nitrogen Mustard Sensitivity and Choline Transport in Walker 256 Carcinosarcoma Cells in Vitro1

supplemented with 10% horse serum. The cells grew ex ponentially with a doubling time of 16 to 17 hours, had a cell volume (mean ±S.E.) of 2313 ±89 cu /tm, and were cloned in soft agar with a cloning efficiency of 68.7 ±6.3%. Sensitivity to the alkylating agent nitrogen mustard (HN2) was comparable to that observed previously with LSI78Y murine lymphoblasts; the D0 (the dose of drug reducing survival to 37% of the initial cell population) was 5.35 ng/ml, and the extrapolation number, n, was 1.27. An investigation of the interaction of hydrolyzed nitrogen mustard and choline transport revealed that uptake of choline-14C obeyed simple Michaelis-Menten kinetics, proceeded "uphill" against a concentration gradient of over 30-fold, and showed a decrease in distribution ratio as choline concentration increased and choline transport was competitively inhibited by hydrolyzed HN2. The Michaelis constant, Km, for choline transport was 2.90 x 10~5M,the transport capacity, Vmttx, was 4.83 x 10~17 moles/min/