Marked increase in PROP taste responsiveness following oral supplementation with selected salivary proteins or their related free amino acids

Abstract

The genetic predisposition to taste 6-n-propylthiouracil (PROP) varies among individuals and is associated with salivary levels of Ps-1 and II-2 peptides, belonging to the basic proline-rich protein family (bPRP). We evaluated the role of these proteins and free amino acids that selectively interact with the PROP molecule, in modulating bitter taste responsiveness. Subjects were classified by their PROP taster status based on ratings of perceived taste intensity for PROP and NaCl solutions. Quantitative and qualitative determinations of Ps-1 and II-2 proteins in unstimulated saliva were performed by HPLC-ESI-MS analysis. Subjects rated PROP bitterness after supplementation with Ps-1 and II-2, and two amino acids (L-Arg and L-Lys) whose interaction with PROP was demonstrated by (1)H-NMR spectroscopy. ANOVA showed that salivary levels of II-2 and Ps-1 proteins were higher in unstimulated saliva of PROP super-tasters and medium tasters than in non-tasters. Supplementation of Ps-1 protein in individuals lacking it in saliva enhanced their PROP bitter taste responsiveness, and this effect was specific to the non-taster group.(1)H-NMR results showed that the interaction between PROP and L-Arg is stronger than that involving L-Lys, and taste experiments confirmed that oral supplementation with these two amino acids increased PROP bitterness intensity, more for L-Arg than for L-Lys. These data suggest that Ps-1 protein facilitates PROP bitter taste perception and identifies a role for free L-Arg and L-Lys in PROP tasting.

Figure 1. Graphic diagram representing the study design.

Figure 2. Relative concentrations of Ps-1 and II-2 in the PROP taster groups in unstimulated (resting) saliva.

Distribution of the XIC peak areas of Ps-1 and II-2 and mean values ± SEM for each taster group are reported. Ps-1 mean values were lower in non-tasters than in PROP super-tasters and those of II-2 were lower in non-tasters relative to the other groups (Ps-1: p = 0.0216; II-2: p≤0.004; Kruskal-Wallis test). Out of 62 subjects, n = 21 non-tasters, n = 17 medium tasters and n = 24 PROP super-tasters. Subjects lacking Ps-1 (n = 20) or II-2 (n = 7) in their saliva are identified by white circles.

Figure 3. Effect of the Ps-1 protein on PROP bitterness intensity.

Mean (± SEM) bitterness intensity evoked by PROP and PROP+Ps-1 solutions (upper graph) in 20 subjects lacking Ps-1. The same data are shown in the lower graph for each taster group (n = 9 non-tasters; n = 7 medium tasters; n = 4 PROP super-tasters). The solution containing only Ps-1 (control) is not shown as it did not evoke any taste perception. * = significant difference (F[_1,17] = 7.2273, p = 0.0155; repeated measures ANOVA). Different letters indicate significant differences (p≤0.0012; Newman-Keuls test subsequent to repeated measures ANOVA).

Figure 4. PROP ring proton ¹H-NMR chemical shift variation upon amino acids addition reported as Δ.

Δ = (|(δ’−δ₀)|/δ₀)·100 represents the absolute value of the difference between the ¹H-NMR signal (ppm) of the PROP ring proton in the absence (δ₀) and in the presence (δ’) of the amino acid of the Ps-1 and II-2 sequences, normalized for δ₀ and expressed as a percentage. For each amino acid, two spectra were recorded in 0.5 mL of 5 mM D₂O solution before and after the addition of an equimolar amount of PROP.

Figure 5. Effect of L-Arg or L-Lys supplementation on PROP bitterness intensity.

Mean values ± SEM of bitterness intensity evoked by PROP, PROP+L-Arg and PROP+L-Lys solutions in a group of 40 subjects (upper graph). The same data are shown in the lower graph for each taster group (14 non-tasters; 14 medium tasters; 12 PROP super-tasters). Control solutions containing only L-Arg or L-Lys are not shown as they did not evoke any taste perception. * = significantly different from PROP (PROP +L-Arg: F[_1,37] = 27.124, p = 0.00001and PROP+L-Lys: F[_1,37] = 5.949, p = 0.0196; repeated measures ANOVA). Different letters indicate significant differences (p≤0.0012; Newman-Keuls test subsequent to repeated measures ANOVA).