Characterization of Mammalian Phosphoribosylglycineamide Formyltransferase from Transformed Cells

Phosphoribosylglycineamide formyltransferase levels were studied in several mammalian tissues and were found to be elevated 3- to 6-fold in some anaplastic cells, including rat hepatoma H-35 and mouse leukemia L1210, as compared to their corresponding normal tissues. The enzyme was found to reside in the cytosol of chicken liver and L1210 cells. The enzyme was purified to near homogeneity, as judged by a single band on polyacrylamide gels after electrophoresis in the presence of sodium dodecyl sulfate, from two mammalian sources, mouse L1210 and mouse Sarcoma 180. Comparison of the digestion patterns of L1210 enzyme and chicken liver enzyme upon exposure to chymotrypsin showed some similarity between the two, as did cross-reactivity in Western blots of the chicken enzyme with antibodies raised to the L1210 enzyme. Subunit molecular weight of the L1210 and Sarcoma 180 phosphoribosylglycineamide formyltransferases is about 117,000. Steady-state kinetics was performed with the purified murine enzyme in the presence of 5'-phosphoribosyl-N-glycineamide and 10-for-myltetrahydrofolate and with 5'-phosphoribosyl-N-glycineamide and 10-formyl-5,8-deazafolate, establishing that these mammalian enzymes utilize 10-formyltetrahydrofolate as the actual cofactor. 11-Formyltetrahydrohomofolate was found to be an inhibitor of the murine and human (HeLa) enzymes, competitive with respect to 10-formyltetrahydrofolate, with K,s of 1 and 3 μM, respectively. 11-Formyldihydrohomofolate was an inhibitor of HeLa cell growth with a 50% effective dose of 8-11 μM.