Validation of edible taste strips for identifying PROP taste recognition thresholds.

The ability to identify bitter tastants in humans depends on a small gene family that encodes approximately twenty-five bitter taste receptors (1,2). These receptors contribute to the large variations in human bitter taste perception (1,3). One important variation in taste perception is the ability to identify the bitter tasting compounds phenylthiocarbamide (PTC) and 6-n-propyl-thiouracil (PROP). These two chemically related thiourea compounds are perceived as bitter by some individuals, but appear tasteless or nearly tasteless to others (4,5). The frequency of tasters for PROP and PTC may vary by both race and ethnicity (3,6,7). The estimated frequency of PROP/PTC tasters among Caucasians is approximately 70% (3,7), while estimated frequencies of PROP/PTC tasters for Chinese, Japanese, and sub-Saharan African populations are generally higher with estimates that vary between 80 and 90% (3,6,7) However, PROP taster status may be less than 50% in some subgroups that have been studied in India (7). Although both compounds exhibit similar properties, PROP is generally used for taste blindness studies because this compound lacks the sulfurous odor of PTC, and does not exhibit the toxicity that is associated with PTC (8,9). The genetics of PROP is considered to be intermediate between a Mendelian trait and a complex trait (1). Although other loci have been proposed (10,11), the TAS2R38 (T2R38) gene on the long arm of human chromosome 7 is thought to be primarily responsible for detecting the bitter taste of both PTC and PROP in humans (1,10,12). Genomic PCR analysis has further demonstrated that the human TAS2R38 gene exhibits single nucleotide polymorphisms at five different sites, which result in seven known haplotypes (1). These haplotypes likely explain the wide variability in PTC and PROP taste perception (1). Each single nucleotide polymorphism encodes a different amino acid in the receptor protein (at amino acid positions 49, 262, 296, as well as 80 (rare), and 274 (rare). Of the seven haplotypes, two predominant forms exist at high frequency outside of sub-Saharan Africa (13). These two major haplotypes are generally responsible for the taster and non-taster phenotypes for PTC and PROP. The predominant taster form of the TAS2R38 receptor contains a proline at amino acid position 49, an alanine at position 262, and a valine at position 296 (the PAV haplotype). In contrast, the most common non-taster form of the receptor expresses an alanine, a valine, and an isoleucine at these same three amino acid positions (the AVI haplotype). Individuals who express either one or two PAV-containing alleles are tasters of PROP and PTC, while individuals who express two AVI-containing alleles are non-tasters of both PROP and PTC (1,13,14). In the absence of the PAV allele, AAI, PVI, and AAV haplotypes may confer intermediate taste sensitivities to both PTC and PROP (9,13). Finally, the two rare haplotypes (ARARI and AHACI) have not been phenotypically characterized. Due to their high frequencies, PAV is considered the major taster haplotype while AVI is considered the major non-taster haplotype. These two predominant haplotypes account for the bimodal distribution for PTC and PROP sensitivity in humans (14). The examination of PROP taster status may be important for human health and nutrition. The ability to taste PROP has been associated with the rejection of other bitter tasting compounds, along with the avoidance of bitter-tasting fruits and vegetables (15). These bitter tasting foods include raw cruciferous vegetables such as cabbage, broccoli and Brussels sprouts (15). Many of these foods are important sources of phytonutrients and antioxidants, both of which may reduce the risk of cancer and other chronic diseases (16). Finally, PTC tasters may have enhanced protection against the overconsumption of bitter tasting dietary goitrogens. These individuals may be less likely to suffer from the formation of goiters in areas where dietary iodine is low, when compared to PTC nontasters (17). However, current laboratory-based methods for assessing PROP sensitivity are not useful for use in large-scale population studies. One limitation to more fully understanding the role of bitter taste and taste blindness in human health and nutrition is the lack of a simple, rapid, and reliable method for characterizing PROP taster status for epidemiological studies. In general, aqueous solutions or impregnated filter papers are used to present PTC or PROP to the test subject (3,8,18). The use of impregnated filter paper may produce unacceptable proportions of false negative responses (8,19), and may also give limited agreement with the classification of tasters when identified by taste recognition thresholds (20). The main goal of this study is to determine the sensitivity of PROP taste recognition thresholds when this thioamide is delivered to the oral cavity via edible taste strips. The development of edible taste strips for both threshold and suprathreshold studies will allow the rapid evaluation of PROP taster status in large populations. This approach will in turn be useful for identifying the proposed role of PROP as a broad indicator of taste perception and food selection in humans (3,21). The use of edible taste strips will further allow the rapid examination of PROP phenotypes for comparison to TAS2R38 haplotypes.