Further observations on the intracellular location and mechanism of action of liver enzymes catalysing the synthesis of l-ascorbic acid

Abstract

The enzyme system catalysing the conversion of D-glu-curonolactone and L-gulonolactone into L-ascorbic acid has been found to be located particularly in the heavier microsomal fractions of the liver homogenates of the rat and goat. Some enzymic activity is also associated with the mitochondrial fraction and evidence has been given to show that this is very likely due to contamination of the mitochondrial fraction by microsomal particles. The microsomes can convert only the lactone forms of D-glucuronic acid and L-gulonic acid into L-ascorbic acid. The methyl and ethyl esters of D-glucuronic acid and L-gulonic acid are converted into L-ascorbic acid only through the intermediate formation of their respective lactones. D-Glucuronic amide and L-gulonic amide are not converted into L-ascorbic acid by the micro-somes. Reduced glutathione and some metal-binding agents enhance the microsomal synthesis of L-ascorbic acid by stimulating the synthesis and not indirectly by protecting the synthesized ascorbic acid. L- Gulonolactone and L-xylohexulonolactone have been identified as the intermediates in the conversion of D-glucuronolactone into L-ascorbic acid in the presence of cyanide by the soluble enzyme preparations from the liver of rat and goat. Both D-glucuronolactone reductase and L-gulonolactone oxidase are absent from the tissues of the primates, guinea pig, Indian fruit bat and red-vented bulbul[long dash]species which cannot synthesize ascorbic acid.