Jay Wiedemann

Date of Award

9-26-2000

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Robert C. Nordlie

Abstract

The enzyme glucose-6-phosphatase (Glc-6-Pase, E.C.3.1.9.3) plays a major role in the maintenance of blood glucose levels. Ehrlich ascites tumor cells pose a stress on the host to maintain blood glucose. Therefore, the Glc-6-Pase and associated substrate/product translocases, must function in response to cancer since the enzyme is common to both the glycogenolytic and gluconeogenic processes. Earlier kinetic studies showed there was a diminished inhibition by exogenous Pi of Glc-6-P hydrolysis in native hepatic microsomes of Tumor-bearing mice compared with controls (Lucius et al. (1995) Biochem. J. 311, 537–540). The first purpose of these studies was to determine the time of onset and of maximal response of the Glc-6-Pase system following the tumor transplantation into mice. The second purpose was to investigate the mechanism behind the tumor-related diminished inhibition of Glc-6-P hydrolysis by exogenously added Pi. Included as investigative tools in these studies were steady-state enzyme kinetics, metabolite transport, and protein immunochemistry. We found that (1) the optimal and stable tumor-response of the Glc-6-Pase system was seen on day 8, (2) the Pi transporter component (T 2β) and the catalytic unit peptides increased and decreased, respectively, in tumor-bearing samples, (3) tumor-bearing microsomal preparations were permeable to Pi and (4) tumors produced changes in the kinetics of T2β. Our observations indicate a specificity of inhibition by Pi at T2β against CP compared with PPi in native microsomes derived from the livers of tumor-bearing mice. Pi does interact with T2β and does get into the native microsomes derived from tumor-bearing mice. This interaction indicates that the diminished P i inhibition of Glc-6-P hydrolysis reported earlier may be manifest at the catalytic unit protein rather than at T2β. In tumor-bearing mice, there is a much increased demand for blood Glc by the developing tumor cells. This heightened demand produces a decrease in blood glucose concentration. This results in a diminished Glc-6-Pase catalytic unit protein and activity because glucose is necessary for its induction. Subsequently, the T2β protein may increase in an attempt to compensate for the diminished catalytic unit protein. These changes in the mouse Glc-6-Pase system in response to tumor cells may serve to favor the biosynthetic activity of Glc-6-Pase, which in turn retains a higher level of Glc-6-P for use in the liver.

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