Lipoprotein that selectively inhibits taste nerve responses to bitter substances

The development of a specific inhibitor for bitter taste has been widely required in the fields of taste physiology and pharmaceutical sciences, but no inhibitor has been available. We found that lipoproteins, PA-LG composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) and PA-LA composed of PA and alpha-lactalbumin (LA) reversibly suppressed the responses of the frog glossopharyngeal nerve to the bitter substances. The frog tongue was treated with PA-LG solution for 10 min and then stimulated by a stimulus dissolved in water. The responses to the bitter substances such as quinine hydrochloride, papaverine hydrochloride, caffeine and L-leucine were completely suppressed by PA-LG, while those to the salt type bitter substances such as CsCl, MgCl2 and tetraethylammonium chloride were not suppressed. The responses to NaCl, galactose, acetic acid and L-alanine were unchanged or only slightly increased. The results suggested that binding of PA-LG to the hydrophobic region of the receptor membranes leads to suppression of the responses to the bitter substances. It was pointed out that PA-LG is useful not only for elucidating the receptor mechanisms of bitter substances, but also can be safely used to mask the bitter taste of foods and drugs, since PA, LG and LA are prepared from foods (soybean and milk).