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.