Potent Inhibition of Human Folylpolyglutamate Synthetase by Suramin

Suramin, a bis-hexasulfonated napthylurea, was studied as an inhibitor of human folylpolyglutamate synthetase (FPGS), a crucial enzyme in folate metabolism. Suramin is a more potent (IC50, 0.9 μM) inhibitor of FPGS partially purified from CCRF-CEM human leukemia cells than is bromosulfophthalein (IC50, 17 μM), the first reported nonsubstrate-analog inhibitor of FPGS (J. J. McGuireet al., Adv. Exptl. Med. Biol.163, 199, 1983). FPGS inhibition by suramin is reversed by bovine serum albumin (which binds suramin). Suramin is a noncompetitive inhibitor with aminopterin (Kii= 0.9 μM;Kis= 1.1 μM) and glutamic acid (Kii= 1.0 μM;Kis= 5.2 μM) as the variable substrates; suramin inhibition tends toward being competitive with respect to the third FPGS substrate, ATP (Kii= 3.4 μM;Kis= 0.35 μM), since the major effect is on itsKm. Suramin is a much less potent inhibitor of two other folate-dependent enzymes, dihydrofolate reductase (IC50, 38 μM; methotrexate (MTX), 0.6 nM) and thymidylate synthase (IC50, 87 μM; MTX, 48 μM). The effects of suramin on growth of CCRF-CEM cells and a MTX-resistant subline (R30dm) expressing low levels of FPGS activity were determined. R30dm is slightly collaterally sensitive to suramin consistent with FPGS inhibition contributing to the cytotoxic mechanism. These data, and those of Rideoutet al.(Int. J. Cancer61, 840, 1995), demonstrating that the reduced folate carrier system of CCRF-CEM is inhibited, suggest that inhibition of folate metabolism could be involved in the mechanism of action of suramin.