Transport and utilization of methyltetrahydrofolates by Lactobacillus casei

Transport of labeled 5-methyltetrahydrofolate (5-methyl-H4PteGlu) and 5-methyl-H4PteGlu5 by Lactobacillus casei was via a carrier-mediated system. Both the natural (l) and unnatural (d) diastereoisomers were transported. Active transport was only demonstrated with the monoglutamate. Comparisons of growth rates with transport rates demonstrated that pteroypolyglutamates were more effectively utilized by L. casei than the monglutamate derivatives. The rate-limiting step in the utilization of higher polyglutamates was their transport into the cell, and in the case of the oxidized polyglutamates, their reduction. Utilization of monoglutamates was not limited by their transport but by metabolism, which also explains the different types of growth response curves observed between monoglutamates and higher polyglutamates. This phenomenon, sometimes called positive drift, prevents normalization of these growth curves. The rate-limiting step with the monoglutamates appears to be either the metabolism of 5-methyl-H4PteGlu or their conversion to polyglutamate forms.