Enzymic synthesis of l-ascorbic acid in different animal species

Abstract

The three enzymes or enzyme systems involved in the biosynthesis of l-ascorbic acid from d-glucuronic acid in animal tissues, catalyzing the following reactions: 1. ( a.) d-glucuronate + TPNH + H + l-gulonate + TPN + 2. ( b.) l-gulonate + DPN +→ [3-keto- l-gulonate] + DPNH + H + 3. ( c.) [3-keto- l-gulonate] → l-ascorbate have been assayed in extracts of various tissues of a number of different animal species. In mammals requiring no dietary ascorbic acid, the liver contains all three enzymes and thus can convert d-glucuronate to l-ascorbate. The kidney contains the first two enzymes but is apparently unable to carry out the last step. In two birds, chicken and pigeon, and in a reptile, the turtle, only the kidney contains all three enzymes. In mammals requiring dietary ascorbic acid, including man, two monkey species, and the guinea pig, the liver is unable to convert l-gulonate to ascorbate, although enzymes catalyzing the first two reactions are present. These findings indicate that the conversion of an oxidation product of l-gulonate, presumably 3-keto- l-gulonate, into l-ascorbate is defective. Failure to form ascorbic acid in the latter species is not caused by an excessively high rate of destruction of l-ascorbic acid.