Acid-induced sweetness of neoculin is ascribed to its pH-dependent agonistic-antagonistic interaction with human sweet taste receptor.
Neoculin (NCL) is a sweet protein that also has taste-modifying activity to convert sourness to sweetness. However, it has been unclear how NCL induces this unique sensation. Here we quantitatively evaluated the pH-dependent acid-induced sweetness of NCL using a cell-based assay system. The human sweet taste receptor, hT1R2-hT1R3, was functionally expressed in HEK293T cells together with G alpha protein. When NCL was applied to the cells under different pH conditions, it activated hT1R2-hT1R3 in a pH-dependent manner as the condition changed from pH 8 to 5. The pH-response sigmoidal curve resembled the imidazole titration curve, suggesting that His residues were involved in the taste-modifying activity. We then constructed an NCL variant in which all His residues were replaced with Ala and found that the variant elicited strong sweetness at neutral pH as well as at acidic pH. Since NCL and the variant elicited weak and strong sweetness at the same neutral pH, respectively, we applied different proportions of NCL-variant mixtures to the cells at this pH. As a result, NCL competitively inhibits the variant-induced receptor activation. All these data suggest that NCL acts as an hT1R2-hT1R3 agonist at acidic pH but functionally changes into its antagonist at neutral pH.
Nakajima
Ken-ichiro
K
Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
Morita
Yuji
Y
Koizumi
Ayako
A
Asakura
Tomiko
T
Terada
Tohru
T
Ito
Keisuke
K
Shimizu-Ibuka
Akiko
A
Maruyama
Jun-ichi
J
Kitamoto
Katsuhiko
K
Misaka
Takumi
T
Abe
Keiko
K
D001218:Aspartame; D031408:Curculigo; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010940:Plant Proteins / Q000737:chemistry*; D013549:Sweetening Agents; D013649:Taste / Q000502:physiology*; D013650:Taste Buds / Q000502:physiology
D010940:Plant Proteins; D013549:Sweetening Agents; C511659:neoculin protein, Curculigo latifolia; D001218:Aspartame