THE SUM OF FLUX CONTROL COEFFICIENTS IN THE ELECTRON-TRANSPORT CHAIN OF MITOCHONDRIA

The sum of the flux control coefficients for group-transfer reactions such as electron transport has been proposed to be two when the coeffi cients are calculated from experiments in which the concentrations of the electron carriers are changed (C-E) but one when they are calculat ed from changes in the rates of the electron-transfer processes (C-ups ilon). We tested this proposal using electron transport in uncoupled b eef heart, potato tuber and rat liver mitochondria. First, with ascorb ate plus N,N,N',N''-tetramethyl-p-phenylenediamine as substrate, the C -E flux control coefficients of ascorbate, N,N,N',N''-tetramethyl-p-ph enylenediamine, mitochondria and oxygen over electron-transport rate w ere measured by direct titration of the concentrations of these electr on carriers. C-E values were close to zero, one, one and zero, respect ively, giving a sum of C-E flux control coefficients of approximately two. At higher concentrations of N,N,N',N'-tetramethyl-p-phenylenediam ine, its C-E control decreased and the sum decreased towards one as pr edicted. Secondly, the C upsilon control coefficients of groups of ele ctron-transfer processes with succinate or ascorbate plus N,N,N',N'-te tramethyl-p-phenylenediamine as substrate were measured. This was achi eved by measuring the effects of KCN (or malonate or N,N,N',N'-tetrame thyl-p-phenylenediamine) on system flux when intermediates were allowe d to relax and on local flux when intermediates were held constant. Th e C-upsilon flux control coefficients were calculated as the ratio of the effects on system flux and on local flux. The sum of the C-upsilon flux control coefficients was approximately one. Whether a sum of one or a sum of two was obtained depended entirely on the definition of c ontrol coefficients that was used, since either sum was obtained from the same set of data depending on the method of calculation. Both defi nitions are valid, but they give different information. It is importan t to be aware of which definition is being used when analysing control coefficients in electron-transport chains and other group-transfer sy stems.