Control of gluconeogenesis by phosphoenolpyruvate carboxykinase in cotyledons of Cucurbita 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 a Ki of 5.9 micromolar. At 25 degrees 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 (CPEPCK) of between 0.7 and 1.0. 3-Mercaptopicolinic acid was a less effective inhibitor of phosphoenolpyruvate carboxykinase at lower temperatures (Ki = 8.6 micromolar at 17 degrees C 13.3 micromolar at 10 degrees C) and had similar effects on the metabolism of [2-14C]acetate by marrow cotyledons when the temperature was reduced to 17 degrees C and 10 degrees 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.