Pyruvate dehydrogenase complex (PDC) activity in human skin fibroblasts appears to be regulated by a phosphorylation-dephosphorylation mechanism, as is the case with other animal cells. The enzyme can be activated by pretreating the cells with dichloroacetate (DCA), an inhibitor of pyruvate dehydrogenase kinase, before they are disrupted for measurement of PDC activity. With such treatment, the activity reaches 5-6 nmol/min per mg of protein at 37°C with fibroblasts from infants. Such values represent an activation of about 5-20-fold over those observed with untreated cells. That this assay, based on [1-14C]pyruvate decarboxylation, represents a valid measurement of the overall PDC reaction is shown by the dependence of 14CO2 production on the presence of thiamin-PP, coenzyme A (CoA), Mg++, and NAD+. Also, it has been shown that acetyl-CoA and 14CO2 are formed in a 1:1 ratio. A similar degree of activation of PDC can also be achieved by adding purified pyruvate dehydrogenase phosphatase and high concentrations of Mg++ and Ca++, or in some cases by adding the metal ions alone to the cell homogenate after disruption. These results strongly suggest that activation is due to dephosphorylation. Addition of NaF, which inhibits dephosphorylation, leads to almost complete loss of PDC activity.
Kwan-Fu Rex Sheu, Chii-Whei C. Hu, Merton F. Utter