Correlation of Liver Cytochrome Oxidase Activity with Mitochondrial Cytochrome Oxidase and Phospholipid Concentrations in Protein-deficient Rats

Abstract

The object of the present study was two-fold: To observe to what extent cytochrome oxidase activity as measured manometrically is correlated with cytochrome oxidase concentration as measured spectrophotometrically and with mitochondrial phospholipid concentration during progressive protein depletion followed by repletion; and to study the influence of methionine fed in a protein-free ration on the maintenance of liver cytochrome oxidase activity. Phospholipids have been shown by others to be necessary for the normal functioning of cytochrome oxidase. Methionine fed in a protein-free ration has been shown to enable the maintenance of certain other liver enzyme and lipid components at higher levels than if methionine is omitted. In protein deficiency, rat liver cytochrome oxidase activity and cytochrome oxidase concentration as measured in isolated mitochondria followed almost identical patterns, both falling to 40 to 50% of normal after 8 weeks and remaining at that level until death of the animals at about 14 weeks. Upon repletion with protein, both activity and concentration of the enzymes returned to normal in a linear fashion within 7 days. Mitochondrial phospholipid concentration follows essentially the same pattern. It appears that, although phospholipid follows the same pattern and is essential for the activity of cytochrome oxidase, the changes in cytochrome oxidase activity are not a reflection of changes in mitochondrial phospholipid in protein deficiency, but more likely they represent a loss of enzyme protein. Addition of methionine to a protein-deficient diet does not protect against loss of cytochrome oxidase, in contrast with its protection of other components of the liver, such as succinic dehydrogenase, ubiquinone, and phospholipids.