Antisweet activity

The sweetness-inhibiting substance ziziphin was isolated from the leaves of the plant Zizyphus jujuba (Rhamnaceae), which is native to China, by Kurihara et al 1 Like gymnemic acids, ziziphin is a glycoside of triterpene that suppresses sweetness in humans. Removal of the acyl group under mild hydrolytic conditions led to complete abolishment of its antisweet activity. 

Figure 5. Scheme for isolation of crude antisweet fraction from the leaves of Ziziphus jujuba. Taste bioassays were performed at each step to ascertain antisweet activity. An X indicates that no antisweet activity was detected. 

The sweetness inhibitory activity of plant terpenoids is evaluated by placing 5 mL of 0.5 or 1 mM solution of the compound in the mouth for 2-3 minutes. On expectorating, the mouth is then washed with distilled water. Then, different concentrations of sucrose (0.1-1 mM) are tasted. The maximum concentration of sucrose at which complete supression of sweetness is perceived may then be recorded for each tastant [133]. In practice, antisweet compounds of plant origin have tended to be ranked in terms of sweetness inhibitory potency by comparison with gymnemic acid I (90) [19]. 

Several natural products are known to modify the sense of sweet taste. Such compounds can be either sweetness inhibitors or sweetness inducers/enhancers. A review of Suttisri et al., 1995 summarizes data on phytochemistry and biological activity of more than 40 triterpenoid sweetness inhibitors based on the oleanane and dammarane skeletons. These saponins were isolated from the leaves of Gymnema sylvestre, Zizyphus jujuba and Hovenia dulcis [224]. Recently, antisweet oleanane-type saponins were isolated from Stephanotis lutchuensis vm. japonic a [225]. 

Some oleanane-type saponins isolated from G. sylvestre exhibited antisweetness acti-vity/ When chewed, the leaves produce a prolonged desensitization effect for sweet and bitter tastes. The sour taste is not affected, while the salty taste is very slightly affected. The saponin fraction, and ethanolic extract of the leaves, showed significant antibacterial and antifungal activities. The aqueous extract of G. hestre possessed hypoglycemic activity. The leaf powder stimulates the heart and the circulatory system, increases the secretion of urine, and activates the uterus. 

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