INTRODUCTION

The axillae of humans contain a large number of apocrine, eccrine, and sebaceous glands. Volatile organic compounds (VOCs) emanating from this area give rise to the canonical axillary odor (commonly referred to as “body odor”). Although this odor is mainly perceived by modern society as unpleasant, several studies indicate that axillary odorants contain chemical signals that affect the menstrual cycle (Preti et al. 2003) or that may be involved in major histocompatibility complex, allele-dependent, mate selection (Wedekind et al. 1995). These studies point to the importance of understanding the factors that contribute to human odor production. Current evidence suggests that genetically based odor profiles play a role in individual identity, immune and stress responses, and that odor may be useful for monitoring physical, emotional, and disease states in humans (Dormont et al. 2013). For this potential to be fully realized, however, careful analytical studies, in well-characterized human populations, are needed. Indeed, recently NIH has urged ethnic/racial diversity and genetic analyses to be inclusion criteria in clinical/medical research (Green et al. 2011). Our approach, herein, is to study the influence of ethnic and genetic factors contributing to human odorant profiles.

Human body odor is a complex mixture of VOCs. Organoleptic and analytical chemistry methodologies have revealed that axillary odor derives from a complex mixture of C2-C11 normal, branched, and unsaturated acids, with the main components being (E)-3-methyl-2-hexenoic acid (E-3M2H) (Pierce et al. 1995; Zeng et al. 1991, 1992, 1996a, b) and 3-hydroxy-3-methylhexanoic acid (3H3M) (Natsch et al. 2003), as well as volatile sulfur compounds, particularly (S)-3-methyl-3-sulfanylhexan-1-ol) (Hasegawa et al. 2004; Natsch et al. 2004; Troccaz et al. 2004). The latter sulfur-containing compounds often are present at very low levels but have high odor impact (i.e., low olfactory threshold). Volatile steroids, such as 5α-androst-16-en-3-one (androstenone) and 5α-androst-16-en-3β-ol (androstenol), originally were thought to play a role in axillary odor (Bird and Gower 1981), but later were found to be minor sensory and analytical contributors relative to the organic acids (Zeng et al. 1996a, b).

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Apocrine secretions, which contain the precursors to axillary odor, are odorless upon secretion and become odorous after interaction with the axillary microbial communities residing on the skin’s surface (Labows et al. 1979; Leyden et al. 1981; Natsch et al. 2003, 2006; Troccaz et al. 2009; Starkenmann et al. 2005; Zeng et al. 1992). In those studies that have collected and used sterile, pure apocrine secretions, the odorless precursors are proteins that contain the odorants within lipid-friendly calyxes (Spielman et al. 1995; Zeng et al. 1992, 1996a). The axillae support a dense bacterial population dominated by two types, Staphylococcus and Corynebacterium. A strong correlation has been found between a dense population of corynebacteria and robust axillary odor production (Barzantny et al. 2012; Leyden et al. 1981).

Recent studies have suggested that human axillary odor is influenced by genetic factors. The human adenosine triphosphate (ATP)-Binding Cassette transporter, sub-family C, member 11 (ABCC11), encodes an ATP-driven efflux transporter that is expressed and localized in human apocrine sweat glands. The ABCC11 protein is known to transport a variety of small molecules (Chen et al. 2005; Kruh et al. 2007) and has been shown to play a role in both the generation of axillary malodor (Baumann et al. 2014; Martin et al. 2010; Preti and Leyden 2010) and earwax (cerumen) phenotype (Toyoda et al. 2009; Yoshiura et al. 2006). A single nucleotide polymorphism (SNP, rs17822931) in ABCC11 (538G→A) leads to a glycine-to-arginine substitution (G180R) in the resulting protein. The G180R variant of ABCC11 results in a dry, white earwax phenotype, which is predominant (80-95%) among East Asian populations (e.g., Japanese, Korean, and Chinese), but is quite rare (0-3%) among individuals of European and African descent. TT homozygous individuals for the G180R variant produce significantly less characteristic axillary odor than both heterozygous (CT) and homozygous (CC) individuals (Harker et al. 2014; Inoue et al. 2010; Martin et al. 2010; Nakano et al. 2009). Previous findings from our laboratory highlighted marked ethnic differences in cerumen VOCs between individuals of Caucasian, East Asian, and African American descent (Prokop-Prigge et al. 2014, 2015). Our current work extends that of previous reports, both ours and that of others, and investigates the influence of ethnicity on the relative amounts of axillary VOCs produced. Based on the pattern of ethnic diversity that exists in ABCC11, we hypothesized that axillary odorants produced by East Asians will differ markedly from those obtained from individuals of European and African descent. Using gas chromatography/mass spectrometry [GC/MS], we examined differences in VOCs found in the axillary secretions of Caucasian, East Asian, and African-American individuals with respect to their ABCC11 genotype.

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