Effect of methotrexate with 5-methyltetrahydrofolate rescue and dietary homocystine on survival of leukemic mice and on concentrations of liver adenosylamino acids. 
We have increased significantly the survival time of DBA/2 mice bearing methionine-dependent L1210 or L5178Y leukemia cells by i.p.  administration of lethal doses of methotrexate (five daily doses of 25 mg/kg body weight) followed by rescue with 5-methyl tetrahydrofolate (five daily doses of 20 mg/kg body weight).  The mice were maintained on a semipurified choline- and cyst(e)ine-free diet containing 0.32% L-methionine.  We further increased significantly the survival time of the treated animals bearing L5178Y cells, but not those bearing L1210 cells, by substitution of 0.86% DL-homocystine for the methionine in the diet.  We have examined the effects of both diets in mice treated with methotrexate and 5-methyl tetrahydrofolate, singly and in combination, on the concentrations of S-adenosylmethionine and S-adenosylhomocysteine in the liver, a tissue highly active in the metabolism of these amino acids.  The substitution of homocystine for methionine in the diet of untreated animals led to a significant increase in S-adenosylhomocysteine and decrease in S-adenosylmethionine in the liver, with a resultant profound decrease in the ratio of S-adenosylmethionine to S-adenosylhomocysteine which was not further altered significantly by administration of methotrexate.