Beta-actin does not show the characteristics of a reference protein in human cortex

Levels of a reference protein must be the same as a proportion of total protein in all tissues and, in the study of human diseases, cannot vary with factors such as age, gender or disease pathophysiology. It is increasingly apparent that there may be few, if any, proteins that display the characteristics of a reference protein within the human central nervous system (CNS). To begin to challenge this hypothesis, we used Western blotting to compare variance in levels of the "gold standard" reference protein, β-actin, in Brodmann's area 9 from 194 subjects to variance of total transferred protein measured as intensity of Ponceau S staining. The coefficient of variance of sum intensity measurements for β-actin levels across all donors was 47% compared to 24 and 27% for the sum intensity of Ponceau S staining measured using two different detection techniques. These data strongly suggest that the level of β-actin, proportional to total protein, is not constant in human cortex which raises further doubt about the use of reference proteins to normalise data in human CNS studies. Considering our data, we suggest an alternative approach to presenting data from Western blotting of human CNS