Polyglutamyl Derivatives of Folate as Substrates and Inhibitors of Thymidylate Synthetase

Abstract

Abstract (l)-Tetrahydropteroyltriglutamate and (l)-tetrahydropteroylhexaglutamate were prepared and tested as substrates for thymidylate synthetase (EC 2.1.1.6) (methylenetetrahydrofolate:deoxyuridylate C-methyltransferase) from Lactobacillus casei. Both tetrahydropteroylpolyglutamates were more effective substrates than (l)-tetrahydropteroylglutamate, enhancing the reaction rate 3-fold when compared at 10-5 m. Pteroylpolyglutamates and their corresponding dihydro and (d)-tetrahydro forms were inhibitors of the enzyme, the inhibitory potency increasing with the number of glutamyl residues. The concentration for 50% inhibition with pteroylglutamate was 1.5 x 10-4 m and for pteroylhexaglutamate 6 x 10-7 m. Inhibition by pteroylhexaglutamate, but not that by pteroylglutamate, was abolished in the presence of 0.4 m NaCl. Inhibition obtained with dihydropteroylhexaglutamate and dihydropteroyltriglutamate (50% at 3.2 x 10-6 m) is sufficient to warrant consideration of these compounds as physiological regulators of thymidylate formation. p-Aminobenzoylhexaglutamate and hexaglutamate did not inhibit thymidylate synthetase at 10-2 m indicating that polyglutamates do not bind to the enzyme in the absence of the pteridine. Dihydropteroyltriglutamate and dihydropteroylhexaglutamate were no more effective than dihydropteroylglutamate as substrates for dihydrofolate reductase (EC 1.5.1.3) (5, 6, 7, 8-tetrahydropteroylglutamate:nicotinamide adenine dinucleotide phosphate oxidoreductase) from L. casei.