The effect of 2-oxoglutarate or 3-hydroxybutyrate on pyruvate dehydrogenase complex in isolated cerebrocortical mitochondria

Metabolism of lactate in the rat brain during the early neonatal period

The metabolism of lactate in isolated cells from early neonatal rat brain has been studied. In these circumstances, lactate was mainly oxidized to CO2, although a significant portion was incorporated into lipids (78% sterols, 4% phosphatidylcholine, 2% phosphatidylethanolamine, and 1% phosphatidylserine). The rate of lactate incorporation into CO2 and lipids was higher than those found for glucose and 3-hydroxybutyrate. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle while scarcely affecting glucose utilization by the pentose phosphate pathway. Lipogenesis from glucose was strongly inhibited by lactate without relevant changes in the rate of glycerol phosphate synthesis. These results suggest that lactate inhibits glucose utilization at the level of the pyruvate dehydrogenase-catalyzed reaction, which may be a mechanism to spare glucose for glycerol and NADPH synthesis. The effect of 3-hydroxybutyrate inhibiting lactate utilization only at high concentrations of 3-hydroxybutyrate suggests that before ketogenesis becomes active, lactate may be the major fuel for the neonatal brain. (-)-Hydroxycitrate and aminooxyacetate markedly inhibited lipogenesis from lactate, suggesting that the transfer of lactate carbons through the mitochondrial membrane is accomplished by the translocation of both citrate and N-acetylaspartate.

Pyruvate dehydrogenase complex is inhibited in calcium-loaded cerebrocortical mitochondria

An impairment of mitochondrial functions as a result of Ca-loading may be one of the significant events that lead to neuronal death after an ischemic insult. To assess the metabolic consequences of excess Ca on brain mitochondria, pyruvate oxidation was studied in isolated cerebrocortical mitochondria loaded with Ca in vitro. The flux of pyruvate dehydrogenase complex (PDHC) [( 1-14C]pyruvate decarboxylation) was inhibited as the mitochondria accumulated excess Ca under the conditions tested: the inhibition in state 3 (i.e., in the presence of added ADP) was greater than in state 4 (i.e., in the absence of added adenine nucleotides). In state 4, the inhibition of the PDHC flux was accompanied by a similar reduction of the in situ activity of PDHC, indicating a change in PDHC phosphorylation. In state 3, the inhibition of the PDHC flux was greater than the corresponding decrease of the in situ PDHC activity. Thus, mechanisms other than the phosphorylation of PDHC might also contribute to the inhibition of pyruvate oxidation. Measurement of PDHC enzymatic activity in vitro indicated that PDHC, similar to alpha-ketoglutarate dehydrogenase complex, was inhibited by millimolar levels of Ca. This observation suggests that PDHC may also be inhibited non-covalently in Ca-loaded mitochondria in a manner similar to that of alpha-ketoglutarate dehydrogenase complex.