Differential sensitivity of normal and leukaemic haemopoietic cells to methionine deprivation by l-methioninase

Abstract

The in vitro sensitivity of bone marrow cells from patients with leukaemia and from patients with non-malignant diseases to l-methionine removal by l-methioninase ( l-methionine-α-deamino-γ-mercaptomethane-lyase, EC 4.4.1.11) was determined using the incorporation of [methyl- 3H]thymidine into acid-insoluble material as an index of survival. When compared with controls growing in medium containing 10 μg/ml of l-methionine, leukaemic cells showed a lower incorporation of [methyl- 3H]thymidine after 24 h in the presence of 0.1 (normal 78 ± 24%; leukaemic 26 ± 18%,p<0.01) or 0.05 (normal 84 ± 15%; leukaemic 50 ± 21%, p<0.01) units of l-methioninase per ml. A similar differential sensitivity of leukaemic cells to l-methioninase was seen after 48 h of incubation. There was little effect on [methyl- 3H]thymidine incorporation in the presence of boiled enzyme. Attempts to reverse l-methioninase toxicity with d-homocystine did not result in a differential effect on the normal cell population. The effects of l-methionine removal with l-methioninase were similar to those observed in l-methionine-depleted culture medium supplemented with 0.1 mM l-homocysteine. After 24 h in such depleted media leukaemic cells showed a lower incorporation of [methyl- 3H]thymidine into acid-insoluble material (normal 88 ± 17%; leukaemic 35 ± 14%, p<0.01) and there was an elevation of the l-methionine-dependent enzymes: methionine adenosyltransferase, tRNA methyltransferase and S-adenosyl-methionine decarboxylase. These results suggest the possibility of trying l-methioninase in the treatment of suitable leukaemias.