Effects of albumin, transferrin and humic-like substances on iron-mediated OH radical formation in human lung fluids.

Inhalation of particulate matter is hypothesized to contribute to health effects by overproducing reactive oxygen species (ROS) and inducing oxidative stress. Fe(II) has been shown to contribute to ROS generation in acellular simulated lung fluids. Atmospheric humic-like substances (HULIS) have been shown to chelate Fe(II) and significantly enhance this ROS generation. Here, we investigate Fe(II)-mediated ^. OH generation from the iron active proteins in lung fluid, albumin and transferrin, and fulvic acid, a surrogate for HULIS, in human bronchoalveolar lavage fluid (BALF). We find that albumin enhances ^. OH generation from inorganic Fe(II) and that transferrin attenuates this enhancement. We estimate the rate constants for albumin-Fe(II) and fulvic acid-Fe(II) mediated O ₂ ^.- reduction (1.9 ± 0.3) M ^-1 s ^-1 and (2.7 ± 0.3) M ^-1 s ^-1 (pH = 5.5, T = 37 °C), 17-25 times the rate for free iron, which we measured to be (110 ± 20) × 10 ^-3  M ^-1 s ^-1 , in agreement with the literature. ^. OH generation measured from fulvic acid-Fe(II) in BALF from 8 individuals with added fulvic acid is successfully predicted rates of ^. OH generation by mixtures of Fe(II), albumin, transferrin, fulvic acid, and ascorbate in saline solution. This indicates that fulvic acid enhances ^. OH formation in BALF, and that albumin and transferrin in BALF moderate the effect. We propose that fulvic acid, and thereby HULIS, is capable of mobilizing Fe(II) away from albumin and transferrin and this increases the formation rate of O ₂ ^.- and ultimately of ^. OH. Furthermore, we find that albumin and transferrin have significantly different impacts on Fe(II)-mediated ^. OH than citrate, a common component of simulated lung fluids, a factor that should be considered carefully in the interpretation of results obtained from solutions containing citrate.
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