P-31 NMR MAGNETIZATION-TRANSFER STUDY OF THE CONTROL OF ATP TURNOVER IN SACCHAROMYCES-CEREVISIAE

P-31 NMR magnetization transfer measurements have been used to measure the steady state flux between P-i and ATP in yeast cells genetically modified to overexpress an adenine nucleotide translocase isoform. An increase in P-i --> ATP flux and apparent ratio of moles of ATP synthe sized/atoms of oxygen consumed (P:O ratio), when these cells were incu bated with glucose, demonstrated that the reactions catalyzed by the t ranslocase and F1F0 ATP synthase were readily reversible in vivo. Howe ver, when the same cells were incubated with ethanol alone, translocas e overexpression had no effect on the measured P-i --> ATP flux or app arent P:O ratio, suggesting that the synthase was now operating irreve rsibly, This change was accompanied by an increase in the intracellula r ADP concentration, These observations are consistent with a model pr oposed for the kinetic control of mitochondrial ATP synthesis, which w as based on isotope exchange measurements with isolated mammalian mito chondria [La Noue, K. F., Jeffries, F. M. H. & Radda, G. K. (1986) Bio chemistry 25,7667-7675].