NASA researchers triggered international headlines in 1996 when they discovered, among
other possible indicators of life, traces of polycyclic
aromatic hydrocarbons (multiringed carbon
molecules found in living cells) along surface fractures in ALH84001.
Once the
molecule is borylated, the boron substituent can be replaced by
other substituents, in this case, by an
aromatic molecule bearing highly soluble tetra (ethylene glycol) chains (TEG).
Certain particle compounds may directly generate ROS in vivo because of their surface chemistry (eg, metals, organic compounds, and semiquinones) or after bioactivation by cytochrome P450 systems (eg, polycyclic
aromatic hydrocarbon conversion to quinones).6, 290 a, 290 b A particle surface or anions present on otherwise more inert particles may disrupt iron homeostasis in the lung and thereby also generate ROS via Fenton reactions.291
Other PM constituents may do so indirectly by the upregulation of endogenous cellular sources (eg, nicotinamide adenine dinucleotide phosphate [NADPH]-RRB- oxidase) 292,293 or by perturbing organelle function (eg, mitochondria) by taken - up PM components.261 Particle stimulation of irritant and afferent ANS fibers may also play a role in local and systemic oxidative stress formation.294 Given the rich antioxidant defenses in the lung fluid, secondarily generated oxidization products of endogenous
molecules (eg, oxidized phospholipids, proteins) or a reduction in endogenous antioxidants per se may be responsible at least in part for the state of oxidative stress in the lungs (along with instigating the subsequent cellular responses) rather than ROS derived directly from PM and its constituents.