Rate-limiting steps in folate metabolism by Lactobacillus casei.

Oxidation of 5-methyltetrahydrofolate to 5,10-methylenetetrahydrofolate was the rate-limiting step in 5-methyltetrahydrofolate metabolism by Lactobacillus casei. The limiting steps in the utilization of suboptimal levels of folate by L. casei were related to the ability of folates to function in purine and/or thymidylate biosynthesis. Folates with glutamate chains of up to at least seven residues were substrates for these biosynthetic enzymes, and comparisons of bacterial growth yields with transport rates for these folates indicated that the polyglutamates were more effective substrates in purine and thymidylate synthesis than the corresponding pteroylmonoglutamates. Lactobacillus casei contained low levels of a B12-independent, pteroylpolyglutamate-specific methionine synthetase. Its methylenetetrahydrofolate reductase also functioned more effectively with pteroylpolyglutamate substrates.