Control of gluconeogenesis by phosphoenolpyruvate carboxykinase in cotyledons ofCucurbita pepo L.

3-Mercaptopicolinic acid, a non-competitive inhibitor of phosphoenolpyruvate carboxykinase (EC 4.1.1.19) was used to study the control of gluconeogenesis by this enzyme in germinating marrow (Cucurbita pepo) cotyledons. In vitro, phosphoenolpyruvate carboxykinase was inhibited by 3-mercaptopicolinic acid, with aKi of 5.9 M. At 25°C the inhibitor caused an increase in the label incorporated from [2-¹⁴C]acetate into CO₂, and a decrease in the label incorporated into the insoluble and neutral fractions. Phosphoenolpyruvate carboxykinase had a flux control coefficient for gluconeogenesis (C_PEPCK^J) of between 0.7 and 1.0. 3-Mercaptopicolinic acid was a less effective inhibitor of phosphoenolpyruvate carboxykinase at lower temperatures (Ki = 8.6 M at 17°C, 13.3 M at 10°C) and had similar effects on the metabolism of [2-¹⁴C]acetate by marrow cotyledons when the temperature was reduced to 17°C and 10°C. The control coefficient for this enzyme did not change with temperature, indicating that phosphoenolpyruvate carboxykinase exerts a high degree of control over gluconeogenesis at all temperatures examined.Abbreviations PEP Phosphoenolpyruvate - PEPCK PEP carboxykinaseThe authors thank Dr. Ian Woodrow (University of Melbourne, Australia) for helpful discussions. This work was supported by a grant from the Science and Engineering Research Council, U.K. (GR/F 50978).