BIOCHEMICAL STUDIES ON PTERIDINES IN PLANTS

II. BIOGENESIS OF FOLIC ACID IN GREEN LEAVES: ENZYMATIC SYNTHESIS OF DIHYDROPTEROIC ACID FROM GUANOSINE COMPOUNDS AND MECHANISM OF ITS SYNTHETIC PATHWAY

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1. Cell-free extracts of Brassica pekinensis Rupr (Hakusai) catalyzed the formation of dihydropteroic acid from guanosine compounds and p-aminobenzoic acid. This reaction required ATP, Mg²⁺ and phosphate.
2. GTP and GDP were more active in dihydropteroic acid synthesis than GMP and guanosine. When GTP was used as substrate, the synthesis was absolutely dependent on ATP and Mg²⁺.
3. 2-Amino-4-hydroxy-6-(D-erythro-1′, 2′, 3′-trihydroxypropyl) dihydropteridine was the most active as a pteridine donor for the formation of dihydropteroic acid. This enzyme reaction needed ATP and Mg²⁺. The L-erythro, D-threo and L-threo isomers of 2-amino-4-hydroxy-6-(D-erythro-trihydroxypropyl) dihydropteridine, 2-amino-4-hydroxy-6-(L-arabo-1′, 2′, 3′-trihydroxy-4′-deoxybutyl) dihydropteridine, 2-amino-4-hydroxy-6-(D-arabo-1′, 2′, 3′, 4′-tetrahydroxybutyl) dihydropteridine and 2-amino-4-hydroxy-6-(D-lyxo-1′, 2′, 3′, 4′-tetrahydroxybutyl) dihydropteridine were tested in the ability of enzymatic transformation to dihydropteroic acid.
4. 2-Amino-4-hydroxy-6-(D-erythro-1′, 2′, 3′-trihydroxypropyl) dihydropteridine seemed to convert to 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine without ATP and Mg²⁺.
5. 2-Amino-4-hydroxydihydropteridine and 2, 4, 5-triamino-6-hydroxypyrimidine were not involved in the synthesis of dihydropteroic acid.
6. 2-Amino-4-hydroxy-6-hydroxymethyldihydropteridine pyrophosphate could be converted into dihydropteroic acid only in the presence of Mg²⁺. The monophosphate ester was not utilized in the conversion.