The properties of folinate and 5-methyltetrahydrofolate (5-CH3-H4PteGlu) transport mechanism of Pediococcus cerevisiae were studied. The uptake was dependent on temperature, pH (optimum for both compounds at pH 6.0), and glucose. Iodoacetate, potassium fluoride, and sodium azide inhibited the uptake. 5-CH3-H4-PteGlu was apparently not metabolized but folinate was metabolized. Metabolism of folinate was reduced by preincubation of cells with fluorodeoxyuridine. The transport system for folinate and 5-CH3-H4PteGlu were specific for the l-isomers. Pteroylglutamate, aminopterin, and amethopterin did not interfere with the uptake. Tetrahydrofolate competed with the uptake of folinate. The transport of folinate and 5-CH3-H4PteGlu at 37 C conformed to Michaelis-Menten kinetics; apparent Km for both compounds was 4.0 × 10−7m, and the Vmax for folinate was 1.0 × 10−10 moles per min per mg (dry weight) and for 5-CH3-H4PteGlu it was 1.6 × 10−10 moles per min per mg (dry weight). Both compounds accumulated in the intracellular pool at a concentration about 80- to 140-fold higher than that in the external medium. Folinate inhibited competitively the uptake of 5-CH3-H4PteGlu with a Ki of 0.4 × 10−7m. Unlike 5-CH3-H4PteGlu, which accumulated only at 37 C, folinate was also taken up at 0 C by a glucose- and temperature-independent process, which was not affected by the metabolic inhibitors mentioned above. Since at 0 C the intracellular concentration of folinate was also considerably higher than the external, binding of the substrate to some cellular component is assumed. The finding of an efficient transport system for l-5-CH3-H4PteGlu is of special interest, since this compound has no growth-promoting activity for P. cerevisiae
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