Synthesis of l-ascorbic acid in rat-liver homogenates: conversion of l-gulono- and l-galactono-γ-lactone and the respective acids into l-ascorbic acid

Abstract

Isolated mitochondria and microsomes from rat liver have been shown to convert L-gulono-gamma-lactone into L-ascorbic acid. Microsomes were 4 times as active as mitochondria and nuclei were without effect. The presence of sulphydryl compounds was essential to prevent oxidation of the L-ascorbic acid. The addition of cyanide, azide, hydroxylamine and carbon monoxide had no appreciable effect on the reaction. 2,4-Dinitrophenol and related nitro-phenols caused a marked stimulation. Treatment of the particulate fraction with deoxycholate gave a "soluble" enzyme which behaved like the original particulate fraction except that the presence of sulphydryl compounds in the enzyme digest was not necessary. Neither the mitochondria, nor the microsomes nor the soluble microsomal enzyme would convert L-gulonate into L-ascorbic acid. However, extracts of rats" liver containing all of the tissue elements could convert L-gulonate into L-ascorbic acid. The formation of L-ascorbic acid from the free acid has been shown to involve an aldonolactonase enzyme not present in the particulate fraction as well as an enzyme in the mitochondria and microsomes. No evidence was obtained that 3-oxogulonic acid" was an intermediate in the conversion of L-gulonate into L-ascorbic acid. A suggested reaction sequence for the conversion of L-gulonate into L-ascorbic acid is: L-gulonate+aldonolactonase enzyme L-gulonyl- enzyme complex[image] L-ascorbic acid.