Physiological concentrations of 2-oxoglutarate regulate the activity of phosphoenolpyruvate carboxykinase in liver.

2-Oxoglutarate was found to inhibit purified rat liver phosphoenolpyruvate carboxykinase when the assay was performed in the direction of either phosphoenolpyruvate or oxaloacetate synthesis. The inhibition was competitive with respect to oxaloacetate or phosphoenolpyruvate, the Ki values being 0.32 +/- 0.04 mM 0.63 +/- 0.19 mM respectively. 2-Oxoglutarate inhibited non-competitively when tested against GTP or Mn2+. The reported cytosolic concentrations of 2-oxoglutarate in rat hepatocytes are such that the enzyme is likely to be significantly inhibited under basal conditions. The cytosolic concentration of 2-oxoglutarate is known to fall precipitously under the influence of glucagon and other hormones that stimulate gluconeogenesis, and it is suggested that the hormone-induced decrease in 2-oxoglutarate content would alleviate the inhibition of phosphoenolpyruvate carboxykinase and stimulate flux from oxaloacetate to phosphoenolpyruvate. The implications of this finding to the rationalization of the role of pyruvate kinase in the stimulation of gluconeogenesis in the fasted state are discussed.

The role of inhibition of pyruvate kinase in the stimulation of gluconeogenesis by glucagon: a reevaluation.

We have reexamined the concept that glucagon controls gluconeogenesis from lactate-pyruvate in isolated rat hepatocytes almost entirely by inhibition of flux through pyruvate kinase, thereby making gluconeogenesis more efficient. 1. We tested and refined the 14C-tracer technique that has previously yielded the opposite conclusion, that is, that inhibition of pyruvate kinase is a relatively unimportant mechanism. The tracer procedure, as used by us, was found to be insensitive to the size of the pyruvate pool, and experiments using modifications of the technique to obviate a number of other potential errors support the earlier conclusion that control of pyruvate kinase is not the predominant mechanism. 2. Any stimulation of formation of glucose that results from inhibition of pyruvate kinase is the consequence of elevation of the steady-state concentrations of phosphoenolpyruvate and all subsequent intermediates in the gluconeogenic pathway. During ongoing stimulation of glucose synthesis by glucagon in isolated hepatocytes, the concentrations of all measured intermediate compounds between phosphoenolpyruvate and glucose were elevated except triose phosphates and fructose 1,6-bisphosphate. The failure of these compounds to rise above control levels indicates that not all gluconeogenic reactions beyond pyruvate kinase were accelerated thermodynamically as would occur with predominant control at pyruvate kinase. We conclude, therefore, that although glucagon inhibits flux through the pyruvate kinase reaction, this does not account for most of the stimulation of gluconeogenesis. Major control sites are also within the pyruvate-phosphoenolpyruvate segment and the fructose 1,6-bisphosphate cycle.

Control of mitochondrial content of adenine nucleotides by submicromolar calcium concentrations and its relationship to hormonal effects.

Rat liver mitochondria were incubated at 30 degrees C with 4 mM ATP in a medium similar in electrolyte composition to that of hepatic cytosol. Under these conditions, a net increase in mitochondrial adenine nucleotides was observed that was dependent on the concentration of free Ca2+ [( Ca2+]) in the incubation medium. At 0.2 microM [Ca2+] or less, there was no demonstrable uptake of adenine nucleotides; at 0.4 microM [Ca2+], or greater, net uptake occurred. The calcium-dependent accumulation of nucleotides by mitochondria required Mg2+ in the incubation medium and was insensitive to carboxyatractyloside. The uptake of adenine nucleotides was enhanced by the addition of antimycin A or antimycin A together with oligomycin. Accumulation of nucleotides appeared to be associated with a small increase in mean mitochondrial volume, but the membrane potential was not affected. No uptake or loss of NAD-NADH by mitochondria was detected. Ruthenium red failed to inhibit the calcium-dependent uptake of adenine nucleotides by the mitochondria, indicating that stimulation of this process by Ca2+ does not involve transport of the cation into mitochondria by the Ca2+ uniporter. Because glucagon acts to elevate cytosolic [Ca2+] from approximately 0.2 microM to 0.6 microM, the same range affecting nucleotide uptake, it is proposed that the increase in mitochondrial adenine nucleotides that follows treatment with glucagon is mediated by the rise in cytosolic [Ca2+] produced by the hormone. This hypothesis was supported by the observation that epinephrine and A23187, agents that raise cytosolic [Ca2+], increased the content of mitochondrial adenine nucleotides in isolated hepatocytes. Furthermore, cells, incubated under calcium-depleting conditions, had a diminished response to glucagon.

Sensitivity of the response of cytosolic calcium in Quin-2-loaded rat hepatocytes to glucagon, adenine nucleosides, and adenine nucleotides.

Hepatocytes from fasted rats were used to study the effect of glucagon on intracellular free cytosolic Ca2+ ([Ca2+]i) indicated by the use of Quin-2-calcium fluorescence. It was found that, in both male and female rats, glucagon increased [Ca2+]i at a half-maximally effective concentration (Kact) of 0.3 nM, a concentration known to be half-maximal for affecting several hepatic functions. Acute chelation of extracellular Ca2+ did not obliterate the hormone effect but shortened its duration. Cyclic AMP, 5'-AMP, ADP, and ATP also increased [Ca2+]i, while adenosine 2':3'-monophosphate and 3'-AMP did not. The rise in [Ca2+]i brought about by glucagon at near physiological concentrations may be responsible for the stimulation of glutamate metabolism produced acutely by glucagon.

The interaction of glucagon treatment and uncoupler concentration on ATPase activity of rat liver mitochondria.

Mitochondria isolated from livers of rats treated briefly with glucagon show an increased ATPase activity in the presence of appropriate concentrations of protonophoric uncouplers (Yamazaki, R. K., Sax, R.D., and Hauser, M.A. (1977) FEBS Lett. 75, 295-299). With the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) the effect of glucagon treatment was most evident at concentrations of uncoupler higher than required for maximal stimulation of ATPase in control mitochondria. In this range of FCCP concentrations that produced the greatest contrast in ATPase activity of control and hormone-stimulated mitochondria, there were no significant differences in delta pH, delta psi, or delta p between the two groups. The presence of added succinate in the ATPase assay system mimicked the effect of glucagon treatment, permitting greater activity at high concentrations of uncoupler without significantly affecting delta p. No significant effect of glucagon treatment or uncoupler concentrations on mitochondrial volumes was observed. Following treatment with glucagon, the mitochondria retained a greater content of Mg+ and K+ throughout the range of FCCP concentrations tested. In the upper range of FCCP concentrations there was appreciable loss of K+ from the mitochondria which was greater in control mitochondria than in mitochondria from glucagon-treated rats or in mitochondria assayed in the presence of succinate. The activity of the uncoupler-dependent ATPase was greatly stimulated by increased concentrations of potassium chloride in the assay medium without significantly diminishing the hormone effect. It is proposed that the intrinsic peptide inhibitor of ATPase is dissociated from the enzyme to an increased degree following glucagon treatment and that high levels of uncoupler inhibit by causing an increased association of the enzyme and its inhibitor.