CONTROL OF GLUCONEOGENESIS BY PHOSPHOENOLPYRUVATE CARBOXYKINASE IN COTYLEDONS OF CUCURBITA-PEPO L

3-Mercaptopicolinic acid, a non-competitive inhibitor of phosphoenolpy ruvate carboxykinase (EC 4.1.1.19) was used to study the control of gl uconeogenesis by this enzyme in germinating marrow (Cucurbita pepo) co tyledons. In vitro, phosphoenolpyruvate carboxykinase was inhibited by 3-mercaptopicolinic acid, with a Ki of 5.9 mu M. At 25 degrees C the inhibitor caused an increase in the label incorporated from [2-C-14]ac etate into CO2, and a decrease in the label incorporated into the inso luble and neutral fractions. Phosphoenolpyruvate carboxykinase had a A ux control coefficient for gluconeogenesis (C-PEPCK(J)) of between 0.7 and 1.0. 3-Mercaptopicolinic acid was a less effective inhibitor of p hosphoenolpyruvate carboxykinase at lower temperatures (Ki = 8.6 mu M at 17 degrees C, 13.3 mu M at 10 degrees C) and had similar effects on the metabolism of [2-C-14]acetate by marrow cotyledons when the tempe rature was reduced to 17 degrees C and 10 degrees C. The control coeff icient for this enzyme did not change with temperature, indicating tha t phosphoelzolpyruvate carboxykinase exerts a high degree of control o ver gluconeogenesis at all temperatures examined.