Isolation and characterization of thaumatin I and II, the sweet-tasting proteins from Thaumatococcus daniellii Benth.

Abstract

From an aqueous extract of the fruit of the tropical plant Thaumatococcus daniellii Benth, two sweet‐tasting basic proteins, here named thaumatin I and thaumatin II, were isolated by ultrafiltration, gel nitration on Sephadex G‐50 and ion‐exchange chromatography on SE‐Sephadex C‐25, using a sodium chloride concentration gradient.The proteinaceous character of the two sweet‐tasting compounds was proven by the characteristic ultraviolet absorption spectrum, the presence of almost 100% polypeptide material as determined by the biuret method, the yield of 100% amino acids on hydrolysis with hydrochlorid acid, the positive reaction with amido black colouring agent (as used in the analysis of the fractions by polyacrylamide gel and starch gel electrophoresis) and by the disappearance of the sweet taste of the compounds after incubation with trypsin.The basic character of the two proteins appears from their isoelectric points of almost 12, as estimated by starch gel electrophoresis at different pH values, and from the precipitation of the proteins at pH 12.The molecules of thaumatin I and II are very similar as shown from their amino acid compositions, which are practically equal; they also have the same N‐terminal amino acid alanine and molecular weights of 21000 ± 600 and 20400 ± 600, respectively (calculated from ultracentrifugal data).The proteins lose their sweetness on heating, on splitting of the disulphide bridges in the molecule and at pH values below 2.5.Thaumatin I and II have an intensely sweet taste, the degree of sweetness being about 1600 times higher than that of sucrose on a Weight basis or 105 times on a molar basis. The threshold values are near 10−4% or 48 nM.