Biochemical micromethods were used for the investigation of changes in mitochondrial oxidative phosphorylation associated with
cytochrome c oxidase deficiency in brain cortex from Mo(vbr) (mottled viable brindled) mice, an animal model of Menkes'
copper deficiency syndrome. Enzymatic analysis of cortex homogenates from Mo(vbr) mice showed an approximately twofold decrease in
cytochrome c oxidase and a 1.4-fold decrease in
NADH:cytochrome c reductase activities as compared with controls. Assessment of mitochondrial respiratory function was performed using
digitonin-treated homogenates of the cortex, which exhibited the main characteristics of isolated brain mitochondria. Despite the substantial changes in respiratory chain
enzyme activities, no significant differences were found in maximal
pyruvate or
succinate oxidation rates of brain cortex homogenates from Mo(vbr) and control mice. Inhibitor titrations were used to determine flux control coefficients of
NADH:CoQ
oxidoreductase and
cytochrome c oxidase on the rate of mitochondrial respiration. Application of
amobarbital to titrate the activity of
NADH:CoQ
oxidoreductase showed very similar flux control coefficients for control and mutant animals. Alternately, titration of respiration with
azide revealed for Mo(vbr) mice significantly sharper inhibition curves than for controls, indicating a more than twofold elevated flux control coefficient of
cytochrome c oxidase. Owing to the reserve capacity of respiratory chain
enzymes, the reported changes in activities do not seem to affect whole-brain high-energy
phosphates, as observed in a previous study using 31P NMR.