Abstract
The enzymatic systems for the biosynthesis and metabolism of folate coenzymes in plants were investigated. The enzymes detected were dihydrofolate reductase, formyltetrahydrofolate synthetase, methenyltetrahydrofolate cyclohydrolase, methylenetetrahydrofolate dehydrogenase, and 5'-phosphoribosyl-5-amino-4-imidazolecarboxamide (AICA-ribotide) formyltransferase. They were widely distributed in higher plants and were partially purified from pea seedlings respectively, and their properties were investigated. Folate antagonists retarded the growth of germinating pea seeds, and also inhibited potently the dihydrofolate reductase purified from pea seedlings. These facts suggest that the formation of folate coenzymes in pea seedlings might be blocked in the reduction of dihydrofolate to tetrahydrofolate. In such"folate-deficient"pea seedlings, AICA-ribotide, known as an intermediate in the biosynthesis of purine nucleotide, was found to be specifically accumulated. This compound was isolated from large amounts of "folate-deficient"pea seedlings and characterized as AICA-ribotide. It was the first case to isolate this compound from higher plants. The results suggest that the biosynthetic pathway of purine nucleotides in plants is similar to that of microorganisms and animal tissues. Furthermore, it was evidenced enzymatically that the AICA-ribotide is metabolized by catalytic action of AICA-ribotide formyltransferase in the presence of 10-formyltetrahydrofolate as coenzyme. 10-Formyltetrahydrofolate is presumably synthesized by two enzyme systems in plants. One is by catalytic action of formyltetrahydrofolate synthetase, and the other by methenyltetrahydrofolate cyclohydrolase and methylenetetrahydrofolate dehydrogenase. Intracellular distribution of these folate-linked enzymes in pea seedlings were investigated. Major portions of their activities were found in the cytoplasmic soluble fraction. A possible pathway for the synthesis and metabolism of folate coenzymes in plants is discussed.
