Measurement of Protein Content in Fruit Juices, Wine and Plant Extracts in the Presence of Endogenous Organic Compounds

Abstract

Measurement of soluble protein in wines, fruit juices and enzyme extracts by the standard Coomassie blue assay gave results that did not agree with protein content calculated from samples and standards on silver-stained SDS-PAGE gels. A simple one-step change has improved the quantitative measurement of proteinThe acidic pH of wines and fruit juices, and acidic to neutral pH of vegetable extracts favour hydrogen bonding of some organic compounds to protein. In a modified Coomassie blue assay the samples were made alkaline to reduce hydrogen bonding of protein to phenolics, fats and their breakdown products. Electrostatic binding to the positively charged guanidino group of arginine, the amino acid responsible for the colour change when protein interacts with Coomassie blue dye, was also reduced. On addition of Coomassie blue dye (pH 0.70) there may be competition for binding to protein by organic compounds and dye, rather than a strengthening of the already hydrogen-bound compounds to protein. Protein content measured by the standard Coomassie blue assay and by alkaline treatment of the samples differed significantly. Results from the ‘alkali assay’ agreed quantitatively with protein estimates from silver-stained SDS-PAGE gels.