Biological synthesis of ascorbic acid: the conversion of derivatives of d-galacturonic acid into l-ascorbic acid by plant extracts

Abstract

Enzymic reduction of derivatives of D-galacturonic acid to derivatives of L-galactonic acid was demonstrated in peas. The enzyme catalyzes a reaction between reduced triphospho-pyridine nucleotide and esters of D-galacturonic acid; the free acid is not reduced. In peas which have been germinated for 2-3 days, the enzyme is located in the soluble part of the cytoplasm, and is absent from the cytoplasmic particles (mitochondria). The formation of L-ascorbic acid from derivatives of D-galacturonic acid thus requires the presence of this enzyme, and the enzyme system within the mitochondria which converts derivatives of L-galactonic acid into L-ascorbic acid. Evidence from chromatographic and kinetic studies suggest that a product of the reduction of methyl D-galacturonate is L-galactono-[gamma]- lactone, although the prior formation of an unstable intermediate is not excluded. The poor conversion of esters of D-galacturonic acid into compounds of L-galactonic acid appears to be due mainly to the reversible inhibition of the enzyme during the course of the reaction, and to a lesser extent to the instability of the galactonic acid derivative formed. The substance responsible for the inhibition of the enzyme was identified. The low affinity of the enzyme for methyl D-galacturonate (Km, 10-2m) suggests that this is not the naturally occurring substrate.