Measurement of protein in tannin‐protein precipitates using ninhydrin

Abstract

Abstract A method was tested for quantifying protein in precipitated tannin‐protein complexes in which protein was hydrolysed with acid and the amino acids released were measured with ninhydrin. Unlike previously published methods, this technique requires no prior separation of tannin and protein and can be used to compare the binding of different soluble proteins to tannins under identical conditions. The method was used to compare the precipitation of bovine serum albumin, porcine pancreatic protease, β‐glucosidase and γ‐globulin by tannic acid. The amount of tannic acid required to precipitate maximal amounts of protease and β‐glucosidase was approximately 7–8 times that required to cause maximal precipitation of albumin and γ‐globulin when tested with 2 mg ml −1 protein. All protein preparations contained a fraction (10–40% of total protein) which was not precipitated by tannic acid. Protein in tannin‐protein precipitates produced in a standard haemanalysis assay were also measured with ninhydrin. Per cent precipitation at each tannic acid concentration as measured with ninhydrin was identical to that determined by haemanalysis within the linear portions of the binding curves produced by the two methods. These results confirm that the ninhydrin method accurately measures precipitation of protein by tannin.