Isolation of the Sugar-binding Protein from Rat Taste Buds

Abstract

DASTOLI and Price¹ have found a protein fraction in bovine tongue extracts that complexes with sugars and saccharin and has some of the properties expected of the sweet taste receptor molecule. A similar protein fraction has also been demonstrated in 0.4–0.6% saturation (NH₄)₂SO₄ fraction of extract from rat tongue, but not in extract from cat tongue, which has little sensitivity to sugars². Also, the order and the relative magnitude of effectiveness of nine sugars on the fraction of extract from rat tongue epithelium, shown by measuring changes in ultraviolet absorbancy of the protein complex with sugars, agree with those obtained electrophysiologically in the rat³. On the other hand, Koyama and Kurihara⁴, after comparing the protein pattern obtained from papillae with taste buds with those from the surrounding epithelium, concluded that a unique protein was not present in taste buds in the bovine tongue. Their inability to find any difference in the protein pattern between extracts from the two kinds of papillae may be due to the minute amount of the specific protein in the taste buds, because their results are not compatible with the finding that suspensions of circumvallate and fungiform papillae bound more ¹⁴C-labelled sucrose than did those from filiform papillae, which are devoid of taste buds⁵. The presence of a minute amount of the sugar-binding protein has been demonstrated at the anodal side of the disc electrophoresis of 0.4–0.6% saturation (NH₄)₂SO₄ fraction of extract from rat tongue epithelium⁶. This sugar-binding protein band was stained by amido black or Coomsian bright and showed a high radioactivity after ¹⁴C-fructose had been added to the fraction.