Biochemistry and Genetics of Human Axilla Odor

Abstract

Human axilla odors are only formed once skin secretions come into contact with the skin microflora. Axilla odors are thus a product of an intricate interplay between skin bacteria and axillary gland secretions. The bacterial populations in the axilla are dominated by Staphylococci and Corynebacteria. The magnitude of odor formation is associated with the population density of Corynebacteria on the skin of human panelists. In recent years, amino-acid conjugates were identified as the key secreted odorant precursors: Different odorant acids are secreted as glutamine conjugates, whereas sulfur volatiles arise from the bacterial degradation of conjugates related to glutathione. Specific enzymes were identified in Corynebacteria cleaving these amino-acid conjugates and thus releasing the odors. Based on this molecular understanding, questions on the evolutionary significance of these odors could be asked. The pattern of secreted odor precursors appears to be stable within an individual and it is genetically determined as was shown in a twin study. Based on behavioral studies, an association between axilla odors and the human leukocyte antigen (HLAhumanleukocyte antigen (HLA)) genotype had been proposed, but the chemical nature of potential HLA-associated body odors remained enigmatic. A family study on siblings with identical HLA-genes could not identify a link between genetically inherited patterns of glutamine conjugates and HLA-types. Axilla odors are largely absent in a significant fraction of the human population in the Far East. This has been associated with a single nucleotide polymorphism (SNPsingle nucleotide polymorphism (SNP)) in the ABCC11 gene. Indeed, human subjects lacking a functional ABCC11 allele are not able to secrete the amino-acid conjugates of the key odorants, which confirm the relevance of these biochemical mechanisms of odor formation identified in the last decade.