Inhibition of phosphoenolpyruvate carboxykinase, glyceroneogenesis and fatty acid synthesis in rat adipose tissue by quinolinate and 3-mercaptopicolinate.

To study the interrelationships of phosphoenolpyruvate carboxykinase and glyceroneogenesis in adipose tissue, investigations with two effectors of the hepatic carboxykinase, Fe2+ and Mn2+, and two inhibitors of the enzyme and of gluconeogenesis in liver, quinolinic acid and 3-mercaptopicolinic acid, were carried out. Incubating adipose tissue cytosol with 30 microM Fe2+ or 100 microM Mn2+ prior to assaying for phosphoenolpyruvate carboxykinase activity doubled the enzyme activity. Inhibition of the enzyme by quinolinate alone was minimal. Adding 30 microM Fe2+ to the cytosol decreased the K0.5 (concentration that gives 50% inhibition) for quinolinate to 0.4 mM and the K0.5 for mercaptopicolinate from 200 to 14 microM. Activating the enzyme with 100 microM Mn2+ did not lower the K0.5 values and adding 500 microM Mn2+ to the cytosol completely interfered with the enhancement of inhibition induced by Fe2+. Each inhibitor interfered with 14C incorporation into glyceride glycerol from labeled pyruvate, alanine and lactate in suspensions of adipocytes. Adding 1 mM Mn2+ to the adipocyte suspension almost completely prevented the inhibition of pyruvate and alanine incorporation into glyceride glycerol, but adding the Mn2+ or 250 microM Fe2+ to the adipocytes in the absence of inhibitors did not enhance glyceride glycerol formation. Adding 250 microM Fe2+ to the adipocytes did not enhance inhibition of lipid synthesis by mercaptopicolinate or quinolinate. Mercaptopicolinate did not inhibit glyceride glycerol, fatty acid, total lipid or CO2 production from glucose. The lack of activation of glyceride glycerol synthesis by added Fe2+ or Mn2+, the lack of enhancement of pyridine carboxylate inhibition by Fe2+ and the interference with inhibition by Mn2+ are compatible with the idea that a transition metal ion similar to Fe2+, if not Fe2+ itself, is available to, or loosely bound to, the adipose tissue carboxykinase in vivo. Taken together with the results of previous work which showed ferroactivator (a cytosol protein necessary for Fe2+ activation of the carboxykinase) to be present in adipose tissue, the present results indicate that the control of the adipose tissue carboxykinase may be similar to the enzyme in liver. Fatty acid synthesis was also diminished by the inhibitors, albeit to a lesser extent than was glyceride glycerol formation. It is hypothesized that this was secondary to decreased esterification caused by the lack of glycerol 3-phosphate from inhibition of the carboxykinase. Decreased esterification would lead to a build-up of fatty acyl CoA which inhibits fatty acid synthesis.