Structural determinants of the hydrogen peroxide permeability of aquaporins

Aquaporins (AQP) conduct small, uncharged molecules, such as water (orthodox AQPs), ammonia (aquaammoniaporins) or glycerol (aquaglyceroporins). The physiological functions of AQPs are involved in osmotic volume regulation or the transport of biochemical precursors and metabolic waste products. The recent identification of hydrogen peroxide (H₂O₂) as a permeant of certain AQPs suggests additional roles in mitigating oxidative stress or enabling paracrine H₂O₂ signalling. Yet, an analysis of the structural requirements of the H₂O₂ permeability of AQPs is missing. We subjected a representative set of wild-type and mutant AQPs to a newly established quantitative phenotypic assay. We confirmed high H₂O₂ permeability of the human aquaammoniaporin AQP8 and found intermediate H₂O₂ permeability of the prototypical orthodox water channel AQP1 from the rat. Differences from an earlier report showing an absence of H₂O₂ permeability of human AQP1 can be explained by expression levels. By generating point mutations in the selectivity filter of rat orthodox aquaporin AQP1, we established a correlation of H₂O₂ permeability primarily with water permeability and secondarily with the pore diameter. Even the narrowest pore of the test set (i.e. rat orthodox aquaporin AQP1 H180F with a pore diameter smaller than that of natural orthodox AQPs) conducted water and H₂O₂. We further found that H₂O₂ permeability of the aquaglyceroporin from the malaria parasite Plasmodium falciparum was lower despite its wider pore diameter. The data suggest that all water-permeable AQPs are H₂O₂ channels, yet H₂O₂ permeability varies with the isoform. Thus, generally, AQPs must be considered as putative players in situations of oxidative stress (e.g. in Plasmodium-infected red blood cells, immune cells, the cardiovascular system or cells expressing AQP8 in their mitochondria).