3-Mercaptopicolinate (3-MP) inhibits d-glucose-6-phosphate (G6P) phosphohydrolase activity of the glucose-6-phosphatase system (Bode et al. (1993) Biochem. Cell Biol. 71, 113-121). We therefore attempted to maximize the inhibition by varying the physical state of microsomes, the concentration of 3-MP, and the time of preliminary incubation of 3-MP with the enzyme. The inhibition was irreversible and time- and inhibitor-concentration-dependent, with G6P phosphohydrolase activity of intact rat liver microsomes, but there was no inhibition with detergent-treated microsomes. The effectiveness of 3-MP as a time-dependent inhibitor of glucose 6-phosphatase was demonstrated in situ by measuring glycogenolysis in isolated, perfused livers from fed rats. We first exposed the livers to 2 mM 3-MP for 40 min, and then assessed the inhibitory effects on glycogenolysis. It was lowered by 50%. These observations establish that 3-MP at the mM level may be useful as an experimental probe in the study of the role(s) of G6P in the regulation of glycogenolysis as well as glycogenesis. Further, they validate the use of much lower (μM) concentrations of 3-MP to block gluconeogenesis (at the phosphoenolpyruvate carboxykinase step) without interfering with glucose 6-phosphatase. We also explored the mechanism of 3-MP inhibition. The time-dependent inhibition of carbamoyl-phosphate:glucose phosphotransferase activity with microsomes incubated with 1 mM 3-MP for 60 or 90 min and then assayed with 1 mM carbamoyl phosphate and 180 mM glucose was modest compared with inhibition of G6P phosphohydrolase. When G6P production by carbamoyl-phosphate:glucose phosphotransferase was reduced by decreasing glucose concentration to 60 mM, no inhibition by 3-MP was discernible. There was no inhibition of inorganic pyrophosphatase activity. These studies support the model of time-dependent, irreversible reaction of 3-MP with the G6P translocase component of the glucose-6-phosphatase system.
|Original language||English (US)|
|Number of pages||7|
|Journal||Biochimica et Biophysica Acta (BBA)/Protein Structure and Molecular|
|State||Published - Oct 19 1994|
Bibliographical noteFunding Information:
This work was supported in part by Research Grant DK 07141 from the National Institutes of Health, U.S. Public Health Service, by National Science Foundation EPSCoR Grant EHR9108770, and by grants from the Dakota and Minnesota Aeries of Eagles and the North Dakota VFW Auxiliary. The authors are grateful to Cathrine E. Snyder for her editorial expertise, and to Bart Pederson, Peggy O'Shea, Virginia Achen, Darby Lowe and Anita Lindquist for their invaluable technical assistance.
- (Rat liver)
- 3-Mercaptopicolinic acid
- Carbamoyl-phosphate:glucose phosphotransferase
- Glucose 6-phosphatase
- Inorganic pyrophosphatase