metabolic consequences of ascorbate biosynthesis in the rat.
Published: 2005
Language: eng
Number of pages: 147
Description: This thesis explores the antioxidant and metabolic consequences of ascorbate biosynthesis in rat hepatocytes. Following chapter 1 (the introduction), chapter 2 explores glutathione (GSH) conjugation and oxidation-induced ascorbate synthesis by identifying 2 GSH-transferase substrates that rapidly deplete GSH and stimulate ascorbate synthesis. In addition, some novel biochemical observations were noted. Ascorbate synthesis was accompanied by rapid glycogenolysis without glucose synthesis and could not be stimulated by the GSH oxidant, diamide. This was contradictory to a past hypothesis that cellular oxidized GSH increased glycogenolysis and thus ascorbate synthesis. Chapter 3 illustrates for the first time that endogenously produced ascorbate can protect against oxidative stress triggered by the organoperoxide, cumene hydroperoxide (CHP). This chapter describes the beneficial, antioxidant activity of endogenous ascorbate synthesis which could supplant the antioxidant role of GSH under oxidative stress conditions. Induction of cellular susceptibility to low concentrations of CHIP was accomplished by depleting GSH. However, the depletion of GSH was also associated with ascorbate synthesis and the synthesised ascorbate was found to significantly protect cells against CHP-mediated reactive oxygen species production, cell death and lipid peroxidation. This ascorbate protective effect was successfully abolished by an inhibitor of ascorbate biosynthesis and did not occur with guinea pig hepatocytes which were unable to carry out ascorbate synthesis. Chapter 4 explores the biochemical changes associated with xenobiotic-induced ascorbate synthesis. This chapter follows from chapter 1 by delineating the potential bioenergetic cost of ascorbate synthesis. High resolution NMR spectroscopy with 13C-labeled glucose was used to identify metabolic fluxes of carbohydrates through glycogenolysis, glycogen synthesis, gluconeogenesis, and glycolysis in the presence of chloretone, a known inducer of ascorbate synthesis. Surprisingly, chloretone caused dramatic decreases in both glycolysis and gluconeogenesis and glycogen synthesis. This was accompanied by a significant increase in 13C labeled ascorbate in the cell and a lowered ability to glucuronidate acetaminophen. These results suggest that the xenobiotic-mediated increase in ascorbate synthesis involves a dramatic redirection of glucose units into the D-glucuronic acid pathway. Since the metabolic regulatory mechanism involves all major pathways of glucose metabolism, the work described in this thesis is also important for non-ascorbate synthesing organisms.
Notes: Thesis (Ph.D.)--University of Toronto, 2005.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 66-06, Section: B, page: 3078.