Abstract
The rat blood:air partition coefficient (PC) of lipophilic volatile organic chemicals (VOCs) cannot be predicted with the sole consideration of their solubility in blood water and lipids, suggesting an important role of blood proteins. The possible concentration dependency and the quantitative nature of VOC binding to blood proteins [i.e., association constant (Ka), number of binding sites (n)] have not been investigated previously. The objectives of this study were therefore (1) to determine the concentration dependency of the blood:air PC (P(b:a)) of four VOCs, bromoform (BF), chloroform (CF), chlorobenzene (CB), and ethylbenzene (EB), hypothesized to display binding to rat blood proteins; and (2) to derive Ka and n values for these chemicals in rat blood. In vitro studies were conducted using 0.1-0.5 ml whole blood, or an equivalent mixture of water and n-octanol exposed to varying amounts of VOCs (BF, 0.11-11.4 micromol; CB, 0.11-24.6 micromol; CF, 0.11-186.6 micromol; and EB, 0.11-20.2 micromol) in sealed glass vials. The P(b:a) of CB, CF, and EB decreased significantly at higher amounts added, with no significant change in their n-octanol + water mixture:air PC. For each in vitro exposure situation, the concentration of free chemical (Cfree) in rat blood was calculated with the PC for the n-octanol + water mixture, whereas the concentration of bound plus free chemical (Ctot) was calculated from knowledge of the PC determined experimentally with whole blood. The respective values of Ka and n for hemoglobin binding estimated by linear regression of a plot of the reciprocal of the molar ratio of bound chemical versus 1/Cfree were: BF, 0.8, 4; CB, 2.8, 1.4; CF, 1.8, 1.2; and EB, 2, 1.4. The results of this study suggest that the concentration-dependent nature of P(b:a) need not be considered for modeling rat inhalation exposures to these VOCs for up to several thousand parts per million.