Contribution of the Translocator of Adenine Nucleotides and the ATP Synthase to the Control f Oxidative Phosphorylation

The regulation of the rate of mitochondrial oxidative phosphorylation and arsenylation was studied at two external free Ca2+ c o n ~ n t r a t i o ~ . The rate of arsenate-stimulated respiration in absence of added ADP was not affected by external and lo-' M Ca2+ levels or carboxyatractyloside, while state 3 respiration was profoundly modified. In addition, the kinetic analysis showed that the rate of arsenylation in the presence of ADP was more efficient (Vm/Km ratio 3.5 times higher) in the catalytic process than phosphorylation. Therefore, this suggests that the activity of the ATP/ADP carrier is importantly controlled by Caz*. The evaluation of the control in phosphorylation showed that the flux-control coefficients (Ci) exerted by the ATP/ADP carrier (ranged between 0.23 and 0.48) and the ATP synthase (0.05-0.57) were modified in a reciprocal way by Ga2+ and Pi concentrations. This suggests that these two enzymes are coupling sequentially through a common intermediate, the intramitochondrial ATP/ADP ratio. Other important steps controlling, p h o ~ h o ~ ~ t i o n were the kl complex (Ci = 0.30) and the cytochrome oxidase (Cj = 0.23) but they were not modified by Ca". It was also found that the main step controlling arsenylation was the ATP synthase (Ci = 0.74). The increment in the inorganic arsenate concentration induced a diminution in the control exerted by the ATP synthase (from 0.73 to 0.56). The results suggest that Ca2' and Pi (or inorganic arsenate) could be regulated by ATP synthesis through an activating effect on ATP/ADS? carrier and/or ATP synthase.