Institute for Clinical and Translational Science

The role of endogenous isoprene in the human body, if any, is unclear because previous research is inconsistent and mechanistic evidence for the biologic function of isoprene is lacking. Given previous evidence that exhaled isoprene is elevated in systemic inflammatory states, we hypothesized that exhaled isoprene would be positively associated with a breath biomarker of airway inflammation, the fractional concentration of exhaled nitric oxide (FE_NO). We examined relationships of exhaled breath isoprene with FE_NO and with outdoor ozone given that ozone chemically reacts with isoprene and has been positively associated with FE_NO in past studies. Sixteen elderly subjects were followed with ≤12 weekly exhaled hydrocarbon and FE_NO collections at the subjects’ retirement community. Outdoor ozone concentrations were measured continuously on site. Mixed-effects regression analyses tested relations of FE_NO with isoprene, and FE_NO and isoprene with ozone, adjusted for temperature. We found FE_NO was inversely associated with isoprene, and this was not confounded by ozone. Isoprene was inversely related to ozone. FE_NO was positively related to ozone and this relation was not confounded by isoprene. In contrast to hypothesized relations, we conclude that exhaled isoprene is inversely associated with FE_NO as well as outdoor ozone, which suggests possible protective ozone-scavenging functions of endogenous isoprene. Findings may indicate chemical reactions of isoprene oxidation by ozone and by hydroxyl radicals in the presence of O₂ that is dependent on NO concentration. These preliminary results need to be confirmed in additional studies of human subjects, particularly as they apply to FE_NO monitoring in asthma.