Abstract
Metabolism of L-ascorbic acid was studied in animal tissues by means of purification of each enzyme in the synthetic and degradative pathways. In the present paper the synthetic pathway is reviewed, and the degradative pathway will be reviewed in the forthcoming paper. Biosynthesis of L-ascorbic acid from D-glucuronic acid requires the following three steps : 1) Reduction of D-glucuronic acid by TPN L-hexonate dehydrogenase to form L-gulonic acid, 2) lactonization of L-gulonic acid by lactonase I (aldonolactonase) to form L-gulonolactone, and 3) oxidation of L-gulonolactone by gulonolactone dehydrogenase to form L-ascorbic acid. TPN L-hexonate dehydrogenase was purified about 500-fold from rat liver supernatant. This enzyme showed a narrow substrate specificity in an oxidation reaction limiting only in L-hexonate and broad specificity in a reduction reaction covering most aldehyde compounds tested. Lactonase I was purified about 60-fold from beef liver extract. This enzyme could catalyze the reversible hydrolysis and formation of a lactone ring by acting on various uronic and aldonic acids. The identity of this enzyme with gluconolactonase and its difference from 6-phosphogluconolactonase were observed. Another kind of lactonase which acts only on glucuronolactone was found in microsomes (lactonase II). Gulonolactone dehydrogenase which is loacted in microsomes catalyzes the oxidation of aldonolactones which have the C_2 levo-configuration. This enzyme is absent in primates and the guinea pig which need a dietary supply of ascorbic acid. To clarify the role of these enzymes in ascorbic acid synthesis, the experiments with tissue slices and a reconstructed system of purified enzymes were performed. In conclusion, ascorbic acid is synthesized in mammalian liver from D-glucuronic acid by way of L-gulonic acid and L-gulonolactone. Only lactonase I among the three lactonases is essential to this process.
