Myeloperoxidase Clearance of Carbon Nanotubes Figure 4. Evaluation of SWCNT size distribution in solubilized lungs of w/t and MPO k/o mice at days 1 and 28 post exposure by transmission electron microscopy. a. Typical TEM images of SWCNT after solubilization of the lung tissue. b. Size distribution of SWCNT present in the lung at days 1 and 28 post exposure. * p,0.05, vs w/t 1 day post exposure, # p,0.05, vs w/t 28 days post exposure. c. Changes in size distribution of SWCNT at day 28 post exposure compared to day 1 post exposure. doi:10.1371/journal.pone.0030923.g004 particles generated by intentional (e.g., during cooking of food or technological high temperature processing of metals and other materials) or unintentional (forest fires, volcano eruptions) exposures. With the advent of nanotechnologies, health concerns prompted exploration of endogenous metabolic pathways compe- tent in clearance of these notoriously biopersistent, inflammogenic and genotoxic, hence potentially high risk materials, including SWCNT. The current work demonstrates that MPO is involved in SWCNT degradation in vivo thus offering new opportunities for controlled regulation of SWNT’s ‘‘life-span’’ in tissues and circulation. Methods Preparation and characterization of SWCNT SWCNT (CNI Inc., Houston, TX) produced by the high pressure CO disproportionation process (HiPco) technique [26] employing CO in a continuous-flow gas phase as the carbon feedstock and Fe(CO)5 as the iron-containing catalyst precursor, and purified by acid treatment to remove metal contaminates [27] were used in the study. Chemical analysis trace metal (iron) in SWCNT was performed at the Chemical Exposure and Monitoring Branch (DART/NIOSH, Cincinnati, OH) using nitric acid dissolution and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Analysis revealed that SWCNT comprised of 0.23 weight % iron. SWCNT were routinely tested for bacterial endotoxin (LPS) contamination using the endpoint chromogenic LAL method, as previously described [28]. The mean diameter and surface area of SWCNT was 1–4 nm and 1040 m2/g. Surface area was determined by Brunauer, Emmett, and Teller (BET) analysis, and diameter and length was measured by TEM. The chemical cutting of SWCNT was performed as reported previously [29]. Purified SWCNT were dispersed in 3:1 mixture of PLoS ONE | www.plosone.org 5 March 2012 | Volume 7 | Issue 3 | e30923