MEASUREMENT OF PROTEIN-CONTENT IN FRUIT JUICES, WINE AND PLANT-EXTRACTS IN THE PRESENCE OF ENDOGENOUS ORGANIC-COMPOUNDS

Measurement of soluble protein in wines, fruit juices and enzyme extra cts by the standard Coomassie blue assay gave results that did not agr ee with protein content calculated from samples and standards on silve r-stained SDS-PAGE gels. A simple one-step change has improved the qua ntitative measurement of protein. The acidic pH of wines and fruit jui ces, and acidic to neutral pH of vegetable extracts favour hydrogen bo nding of some organic compounds to protein. In a modified Coomassie bl ue 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 am ino acid responsible for the colour change tc,hen protein interacts wi th Coomassie blue dye was also reduced On addition of Coomassie blue d ye (pH 0.70) there may be competition for binding to protein by organi c compounds and dye, rather than a strengthening of the already hydrog en-bound compounds to protein. Protein content measured by the standar d Coomassie blue assay and by alkaline treatment of the samples differ ed significantly. Results from the 'alkali assay' agreed quantitativel y with protein estimates from silver-stained SDS-PAGE gels. (C) 1997 A cademic Press Limited.