Abstract
The pyruvate dehydrogenase complex (PDHc) converts pyruvate produced during glycolysis of glucose to acetyl-coenzyme A (acetyl-CoA). The activated acetyl unit is oxidized completely to CO2 by the enzymes of the TCA cycle; reportedly the PDHc is not active in termites. Rather, termites utilize acetate that is produced from the pyruvate by microorganisms in the hindgut and then transferred into the body of the termite through the hindgut epithelium. Acetate is converted to acetyl-CoA in the cytosol of termite cells and the acetyl unit is transported into the mitochondria to be oxidized to CO2 via the TCA cycle.
In this study, the activities of the PDHc were determined in extracts from tissues of Coptotermes formosanus by measuring directly the [14C]-acetyl-CoA produced in reactions between [2-14C]-pyruvate, CoA, and NAD+ catalyzed by the enzyme complex. It was found that an inhibitor cocktail containing 0.1 mM fluphenazine and 0.1 mM dichloroacetophenone effectively inhibited activation and deactivation of the PDHc in termite tissue mitochondria and allowed the determination of the range of PDHc activity. PDHc activity was maintained at about 50-60% of its maximal value in both Nasutitermes walkeri and C. formosanus. Oxygen consumption by mitochondria isolated both from N. walkeri and from C. formosanus was then measured with oxygen electrodes. Pyruvate and acetyl groups (transported into mitochondria as acetylcarnitine or acetyl-CoA + carnitine) were the major respiratory substrates in mitochondria from both N. walkeri and C. formosanus. Sufficient PDHc activity and the high rate of pyruvate oxidation in mitochondria from N. walkeri suggest that pyruvate is rapidly metabolized, whereas the low mitochondrial PDHc activity of C. formosanus suggests that in this species more pyruvate is produced than can be oxidized in termite tissues.
