CONTROL OF THE EFFECTIVE P O RATIO OF OXIDATIVE-PHOSPHORYLATION IN LIVER-MITOCHONDRIA AND HEPATOCYTES/

The control exerted by substrate oxidation reactions, by ATP turnover and by the proton leak over the oxygen consumption rate, the phosphory lation rate, the proton leak rate and the protonmotive force (DELTAp) in isolated rat liver mitochondria under a range of conditions between non-phosphorylating (State 4) and maximum phosphorylation (State 3) w as investigated by using the top-down approach of metabolic control an alysis. The experiments were carried out with saturating concentration s of the substrates succinate, glutamate with malate, or pyruvate with malate. The distribution of control was very similar with each of the three substrates. The effective P/O ratio (i.e. not corrected for lea k reactions) was also measured; it varied from zero in State 4 to 80-9 0% of the maximum theoretical P/O ratio in State 3. Under most conditi ons control over the effective P/O ratio was shared between proton lea k (which had negative control) and the phosphorylating subsystem (whic h had roughly equal positive control); near State 4, substrate oxidati on reactions also acquired some control over this ratio. In resting he patocytes the effective P/O ratio was only 50% of its maximum theoreti cal value, corresponding to an effective P/O ratio of only 1.3 for com plete oxidation of glucose. The effective P/O ratio for intracellular mitochondrial oxygen consumption was 64% of the maximum value. The con trol coefficient of the mitochondrial proton leak over the effective P /O ratio in cells was -0.34; the control coefficient of phosphorylatio n reactions over this ratio was 0.31 and the control coefficient of su bstrate oxidation reactions over the ratio was 0.03, showing how the c oupling efficiency in cells can respond sensitively to agents that cha nge the proton leak or the ATP demand, but not to those that change su bstrate oxidation.