An accurate method for the quantification of cytochrome C oxidase in tissue sections

Cytochrome oxidase (COX) is the enzyme that constitutes the last step of the mitochondrial electron transport chain for the production of ATP. Measurement of COX activity can be achieved by histochemistry, thus providing information about the metabolic status of the brain. Brain regions with high metabolism will present high COX activity in histochemistry assays and vice versa. Using histochemistry versus biochemistry to assess COX activity presents the advantage of providing a map of the differences in metabolism in discrete brain regions. Moreover, COX histochemistry allows quantifying the activity of a particular brain region, by converting units of optical density into units of activity. In the present work we have devised a methodology that allows not only quantifying differences in COX activity between groups, but also quantifying the amount of COX present in brain tissue sections, by directly relating optical density (OD) measurements to cytochrome C oxidase concentration, something that traditionally is achieved by the use of western blot. For this purpose we created a set of standards of known concentration of COX that were affixed to a nitrocellulose membrane, and this membrane was incubated together with the tissue sections in which COX activity was assessed. A standard curve was created using a gradient of different concentrations of purified bovine heart cytochrome oxidase (from 2μg to 0.1μg in intervals of 0.25μg). This standard curve allowed us to detect changes in optical density as low as 5%, and relate these OD differences with known concentrations of cytochrome C oxidase.