The metabolism of methotrexate in Lactobacillus casei and rat liver and the influence of methotrexate on metabolism of folic acid.

[8H]Methotrexate was metabolized mainly to 4-amino-4deoxy-lo-methylpteroic acid in resting cells of ZactobaciZZus casei and a small amount of methotrexate remained inside the cells after 24 hours of incubation. Preincubation of L. casei with methotrexate inhibited the incorporation of [3H]folic acid completely and I-5-formyl-[3H]tetrahydrofolic acid partially into pteroyloctaglutamate and increased the amount of pteroyltetraglutamate and lower polyglutamates. In rats, methotrexate was metabolized partially to lower methotrexate polyglutamates ; however, its uptake was much lower than folic acid and no pentaglutamate of methotrexate was found. Pretreatment of rats with 100 fig of methotrexate daily for 5 days decreased the uptake of i3Hlfolic acid into liver considerably (-20 to -8 %) and the radioactivity in liver was mainly reduced derivatives of pteroylpentaglutamate. With treatment with 300 pg of methotrexate per day for 6 days, however, not only the uptake of [3H]folic acid decreased to -4 %, but also pteroylpentaglutamate comprised only -50% of total radioactivity in liver and the rest was lower polyglutamate and unmetabolized m0noglutamate. Column chromatography with Sephadex G-25 and DEAEcellulose provides a good method for the identification of methotrexate-related compounds.