Elevation of the Blood Glucose Level is Involved in an Increase in Expression of Sweet Taste Receptors in Taste Buds of Rat Circumvallate Papillae.

The gustation system for sweeteners is well-known to be regulated by nutritional and metabolic conditions, but there is no or little information on the underlying mechanism. Here, we examined whether elevation of the blood glucose level was involved in alteration of the expression of sweet taste receptors in circumvallate papillae (CP) and sweet taste sensitivity in male Sprague-Dawley rats. Rats under 4 h-fed conditions following 18 h-fasting exhibited elevated blood glucose levels and decreased pancreatic T1R3 expression, compared to rats after 18 h-fasting treatment, and they exhibited increased protein expression of sweet taste receptors T1R2 and T1R3 in CP. Under streptozotocin (STZ)-induced diabetes mellites (DM) conditions, the protein expression levels of T1R2 and T1R3 in CP were higher than those under control conditions, and these DM rats exhibited increased lick ratios in a low sucrose concentration range in a brief access test with a mixture of sucrose and quinine hydrochloride (QHCl). These findings indicate that the elevation of blood glucose level is a regulator for an increase in sweet taste receptor protein expression in rat CP, and such alteration in STZ-induced DM rats is involved in enhancement of their sweet taste sensitivity.

Designing antioxidant peptides based on the antioxidant properties of the amino acid side-chains.

Amino acids exert characteristic antioxidant activities depending on the properties of their side residues. The hydrophobic residues were effective against peroxyl radical, while acidic residues and their analogs were effective against peroxynitrite. Peptides containing tyrosine showed different activities against different reactive oxygen species (ROS) and/or reactive nitrogen species (RNS). The number and position of tyrosine did not affect the antioxidant activity against hypochlorite ion. Against the peroxyl radical, the number of tyrosine residues affected the antioxidant activity, while its position did not have a significant effect. The tyrosine position was an important factor for the antioxidant activity against peroxynitrite. The peptide GWWW showed higher antioxidant activity against peroxyl radical than tryptophan at concentrations below 25 µM, and high activity against peroxynitrite at 250 µM. Our results suggest that antioxidant peptides against a specific target ROS or RNS can be designed based on the characteristics of the amino acid side chains.