Biosynthesis of folic acid compounds in plants VI. The occurrence and properties of the dihydrofolate-synthesizing enzyme in pea seedlings<xref ref-type="fn" rid="fn1"><sup>1</sup></xref>

Abstract

An enzyme, which catalyzes the formation of dihydrofolate from dihydropteroic acid and L-glutamic acid, was found in pea seedlings. The enzyme was purified approximately 25-fold from the crude extracts of pea seedlings, and its some properties were investigated. Optimum pH for the enzyme activity was found to be 8.8. Pteroic and tetrahydropteroic acids were not active as substrate. The enzymatic reaction required as cofactors ATP, divalent (Mg2+ or Mn2+) and univalent (K+, NH4+ or Rb+) cations. The product was characterized as dihydrofolic acid by bioautography. MICHAELIS constants for L-glutamic acid, ATP, dihydropteroic acid and Mg2+ were 7.0×10−4, 9.0×10−5, 3.5×10−6 and 1.2×10−3 M, respectively. The MICHAELIS constant for Mn2+ was 3.0×10−4. The enzyme was inhibited by PCMB or silver nitrate and, to some extent, by L-aspartic acid. Inhibition by PCMB was completely reversed by addition of 2-mercaptoethanol. Enzyme activity was distributed widely among plants. The importance of magnesium and potassium ions for enzyme catalysis is discussed.