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Abstract
The enzymatic degradation of carbon nanotubes (CNTs) by several enzymes has been reported. However, because organisms that possess these enzymes have limited habitats and distribution areas, it is unclear whether CNTs can be degraded in the general environment. The investigation of CNTs degradation by enzymes derived from bacteria, which inhabit a wide range of environments and have diverse metabolic systems, is inevitable for predicting the environmental fate of CNTs. In this study, the degradation of oxidized (carboxylated) single-walled CNTs (O-SWCNTs) by mt2DyP, a dye-decolorizing peroxidase of Pseudomonas putida mt-2, a common soil bacterium, was investigated. Suspensions of O-SWCNTs gradually became transparent and their optical absorbance decreased during 30 d of incubation in the presence of mt2DyP produced by a recombinant Brevibacillus choshinensis strain and its substrate, H2O2. The degradation was enhanced by higher H2O2 concentrations. The measurement of Raman spectra revealed the complete degradation of O-SWCNTs after 30 d of incubation with 100 mM H2O2. However, surprisingly, this heme enzyme was inactivated within 60 min of the incubation with O-SWCNTs, which suggested that the degradation of O-SWCNTs was not catalyzed by the enzyme. The inactivation of mt2DyP was accompanied by the release of iron, which suggested that the degradation of the O-SWCNTs was owing to the Fenton reaction caused by the iron released from mt2DyP and the supplied H2O2. A chelating agent, diethylenetriaminepentaacetic acid, significantly inhibited the O-SWCNTs degradation, proving the degradation by the Fenton reaction. These phenomena were also observed with another heme enzyme, Cytochrome P450. These results are important for predicting the fate of CNTs in a wide range of environments, as heme enzymes are secreted by many bacteria in the environment. This study also shows that the effect of the Fenton reaction should be considered to validate the degradation of CNTs by heme enzymes.
