BIOCHEMICAL STUDIES ON PTERIDINES IN PLANTS

Abstract

The main object of this investigation was an attempt to elucidate the steps and intermediates comprising biosynthetic pathway of dihydropteroic acid in a higher plant, Brassica pekinensis Rupr. The inhibition tests with three types of inhibitors specific to affect the respective different processes were projected for this purpose. The results gained as itemized in the followings offered a powereful support for the suggestion that the pathway originating in GMP might proceed through the three steps blocked by the inhibitors, including as intermediary precursors 2-amino-4-hydroxy-6-(D-erythro-trihydroxypropyl) dihydropteridine, 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine and the pyrophosphate ester of the latter pteridine. The cell-free system of the plant was able to catalyze the formation of dihydropteroic acid not only from GMP but also from these pteridine derivatives. These conversions required ATP, Mg2+ and p-aminobenzoic acid, except the case of the pyrophosphate compound that could be immediately transformed to the final product without ATP.1. When 2-amino-4-hydroxy-6-(D-erythro-trihydroxypropyl) dihydropteridine, 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine and the pyrophosphate ester of the latter pteridine were tested as substrates, the formation of dihydropteroic acid from these pteridines was inhibited by p-aminosalicylic acid, sulfanilic acid and sulfanilamide, These findings suggested that the donors are metabolized to dihydropteroic acid by coupling of the phosphate ester with p-aminobenzoic acid.2. The existence of 2-amino-4-hydroxy-6-carboxydihydropteridine, 2-amino-4-hydroxydihydropteridine and dihydrobiopterin caused significant inhibition of dihydropteroic acid formation derived from both 2-amino-4-hydroxy-6-(D-erythro-trihydroxypropyl) dihydropteridine and 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine, but not from the pyrophosphorylated derivative of the latter substance. The most powerful inhibition was shown by the carboxydihydropteridine and reversed by increasing the concentration of the hydroxymethyldihydropteridine supplied in the media. These data indicated a possibility that the inhibitors might affect the pyrophosphorylation of the hydroxymethyldihydropteridine by action of ATP and the transformation of the trihydroxypropyldihydropteridine to the final product might involve this step.3. When 2-amino-4-hydroxy-6-(D-erythro-trihydroxypropyl) dihydropteridine was examined together with one of the corresponding L-erythro, D-threo and L-threo isomers, the yield of dihydropteroic acid was reduced, while no inhibition resulted by the combination of the isomers with either 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine or this pyrophosphorylated pteridine. L-Erythro form was most effective as inhibitor and the inhibition induced by this isomer was partially released by increasing the amount of the D-erythro derivative applied. From these evidence, it was assumed that the isomer could act on the conversion process of the D-erythro compound to the hydroxymethyldihydropteridine.4. Sulfanilic acid, 2-amino-4-hydroxy-6-carboxydihydropteridine and 2-amino-4-hydroxy-6-(L-erythro-trihydroxypropyl) dihydropteridine blocked the production of dihydropteroic acid from GMP. This finding provides a reasonable basis for an assumption that the pathway of GMP metabolized to the final product might involve the three steps suggested above.5. The present findings are discussed in relation to other recent works on pteridine biosynthesis.