Control of mitochondrial respiration. The contribution of the adenine nucleotide translocator depends on the ATP- and ADP-consuming enzymes.

The consequence of the complexity of the metabolic network on the amount of control strength of adenine nucleotide translocator was investigated with isolated rat liver mitochondria. Two experimental systems were compared: (i) mitochondria in the presence of yeast hexokinase (hexokinase system) and (ii) the same system plus additional pyruvate kinase (pyruvate kinase system). In both systems the control strength was analysed for the adenine nucleotide translocator by inhibitor titration studies with carboxyatractyloside and for the hexokinase or pyruvate kinase by changing their relative activities. Experimental results were compared with computer simulation of these systems and that of a third one, where the extramitochondrial ATP/ADP ratio was held constant by perifusion (perifusion system). The results demonstrate quite different flux-dependent control strength of the translocator in the three systems. In the hexokinase system the control strength of the translocator on mitochondrial respiration was zero up to respiration rates of about 60 nmol O2/mg protein per min. For higher rates, the control strength increased until the maximum value (0.45) was reached in the fully active state. Here, the same value was also found in the pyruvate kinase system. In all other states of respiration the translocator exerts a higher control strength in the pyruvate kinase system than in the hexokinase system. This different behaviour was attributed to the various changes in the adenine nucleotide pattern caused by partial inhibition of the translocator in the hexokinase and pyruvate kinase system. The data clearly show that the sharing of control strength depends not only on the respiration rate but also on the complexity of the metabolic system.