Studies in the Folate Metabolism of Rats

Abstract

The metabolism of 2-14C folic acid was studied in three-week-old weanling Wistar rats. Distribution of radioactivity after an oral dose of 2-14C folic acid was determined in urine, faeces, liver, and gut of the animals. Some of the urinary metabolites were characterized by column chromatography. The effects of an oral dose of methotrexate on folic acid metabolism in adult male Wistar rats were studied. An oral dose of 10 mg/kg body weight methotrexate was administered 24 hours before 2-14C folic acid (76 microgram/kg body weight) dosing. The urinary metabolites characterized by column chromatography were compared and evaluated, quantitatively and qualitatively, with those present in the urine of control animals. Similarly, the metabolism of 3'-5'-9 ³H folic acid was studied in the presence of methotrexate (10 mg/kg and 100 mg/kg) in rats. A large amount of unmetabolized folic acid was found in the urine of methotrexate-treated animals. 10-Formyltetrahydrofolate and p-aminobenzoyl-L-glutamate were the other major metabolites present in the urine. The major metabolite in the urine of control rats was characterized as 5-methyltetrahydrofolate. The metabolism of 5-14C methyltetrahydrofolate was also studied. Since commercial 5-methyltetrahydrofolate was labelled with ¹⁴C at the labile methyl group, the metabolism was also studied using 5-methyltetrahydrofolate, isolated and purified from the urine of rats dosed with 2-14C folic acid. The effect of methotrexate on 2-14C folic acid and 3'-5'-9 ³H folic acid metabolism was studied in the rats transplanted with 256 Walker carcinosarcoma. It was concluded that methotrexate inhibits purine biosynthesis by interfering with the steps involving formylfolate. This inhibitory action of methotrexate is in addition to the inhibition of dihydrofolate reductase. In view of this, the effects of 6-mercaptopurine, a classical purine inhibitor, on folate metabolism were studied. In addition to the characterization of urinary metabolites, an effort was made to identify the nature of metabolites present in the liver throughout the experiments. The main introduction describes the historical background of folates, methotrexate, and 6-mercaptopurine.