Regulation of ascorbate synthesis in murine liver

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Abstract

Liver plays a central role in the synthesis and export of the two main water soluble antioxidants, glutathione (GSH) and ascorbic acid. In vivo studies have shown that GSH deficiency induces ascorbate synthesis in mouse liver. More recently we have reported that ascorbate production is connected to hepatic glycogen; agents provoking glycogenolysis (glucagon, dibutyryl cAMP, okadaic acid, vasopressin, phenylephrine) simultaneously stimulate ascorbate synthesis both in vivo and in isolated mouse hepatocytes. In the present study a possible link between these two phenomena was investigated. GSH deficiency caused by various agents increased ascorbate synthesis with a simultaneous elevation of glycogen breakdown in isolated hepatocytes. Addition of UDP-glucose to permeabilised hepatocytes or gulonolactone to intact cells increased ascorbate synthesis independently of glutathione depletion. Inhibition of glycogenolysis by fructose abolished the stimulation of ascorbate synthesis caused by increased GSH consumption. Reduction of oxidised glutathione by dithiothreitol prevented the elevation of glycogenolysis and ascorbate synthesis in diamide or menadione treated hepatocytes highlighting the effect of oxidised glutathione on glycogenolysis. Stimulation of ascorbate synthesis by gulonolactone decreased the GSH level in hepatocytes. Inhibition of catalase by aminotriazole deepened the effect. In hepatic microsomes ascorbate synthesis from gulonolactone also caused the consumption of added GSH. This effect could be moderated by the addition of catalase or the glutathione peroxidase inhibitor mercaptosuccinate indicating the role of hydrogen peroxide formed during ascorbate production in GSH depletion. Our results suggest that glutathione and ascorbate are linked not only via redox coupling but also via metabolic connections. The change of GSH/GSSG ratio proves to be sufficient precondition of altering glycogenolysis and the consequent glycogenolysis-linked ascorbate synthesis. On the other hand, enhanced ascorbate production is accompanied by GSH consumption in the liver. This effect may be one of the reasons why the loss of the ascorbate synthesising ability of primates could be advantageous.

Original languageEnglish
Pages (from-to)524S
JournalBiochemical Society transactions
Volume24
Issue number4
DOIs
Publication statusPublished - 1996

ASJC Scopus subject areas

  • Biochemistry

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