Regulation of pyruvate dehydrogenase complex activity in Ehrlich ascites carcinoma cells by Ca2+ and pyruvate

The dependence of pyruvate dehydrogenase complex (PDC) activity on [Ca2+] w as determined in Ehrlich ascites carcinoma cells at different pyruvate conc entrations. The resulting family of curves had the following characteristic s: a) bell-shaped appearance of all curves with maximum activity at 600 nM Ca2+; b) unchanged position of maxima with changes in pyruvate concentratio n; c) nonmonotonous changes in PDC activity with increasing pyruvate concen tration at fixed [Ca2+]. Feasible mechanisms involving Ca2+-dependent phosp hatase and kinase which are consistent with the experimental findings are d iscussed. To determine the steps in the chain of PDC reactions which determ ine the observed phenomena, a mathematical model is suggested which is base d on the known data on the structural-functional relationships between the complex components-pyruvate dehydrogenase (E1), dihydrolipoyl acetyl transf erase (E2), dihydrolipoyl dehydrogenase (E3), protein X, kinase, and phosph atase. To adequately describe the non-trivial dependence of PDC activity on [Ca2+] at different pyruvate concentrations, it was also necessary to cons ider the interdependence of some steps in the general chain of PDC reaction s. Phenomenon (a) is shown to be due only to the involvement of protein X i n the PDC reactions, phenomenon (b) to be due to changes in the activity of kinase, and phenomenon (c) to be due to dependence of acetylation and tran sacetylation rates on pyruvate concentration.