Salvage of methyletrahydrofolate from host cells and methionine biosynthesis in P. Falciparum

Abstract

Plasmodium falciparum P.knohlesi and P.chabaudi show low but detectable activity of methylenetetrahydrofolate reductase (MTHFR), which normally catalyzes the reduction of 5,10-methylenetetrahydrofolate to methyl-tetrahydrofolate. The enzyme was detected by the reverse reaction which radiolabeled methyltetrahydrofolate was used as a substrate and menadione act as artificial electron acceptor. The product, methylenetetrahydrofolate, dissociated spontaneoulsy to tetrahydrofolate and radiolabeled fomaldehyde. The fomaldehyde was trapped as dimedone adduct and was measured by liquid scintillation counter. The presence of this enzyme completes the methionine synthesis cycle, in which the one-carbon fragment from serine side-chain can be transferred to methionine. However, the possible metabolic significance of this cycle in P.falciparum could not be demonstrated by metabolic labellng of methlonine by L-3[14)C] serine. The fallure to demonstrate serine converslon to methlonine is posslble due to the limitation in sensitivity of the assay system. By contrast, the physiological activity of the enzyme could be demonstrated by an aernatiye pathway in P.falciparum for salvage of exognous 5-methyltetrahydrofolate from host cell. The methyl group of the taken up cofactor was incorporated into methionine. Furthermore, it could also demonstrate that the salvaged cofactor was metabolized tothe final product, methyltetrahydropteroylpentaglutamate, which is the same pool as the cofactor derived from de noro blosyrthesis started from p-aminobenzoic acid.