Comparison of styrene-7,8-oxide adducts formed via reaction with cysteine, N-terminal valine and carboxylic acid residues in human, mouse and rat hemoglobin.

The reactive metabolite of styrene, styrene-7,8-oxide (SO), reacts with a variety of nucleophilic sites in hemoglobin (Hb) to form SO-Hb adducts. Following the in vitro incubation of SO with blood from humans, NMRI mice and Sprague-Dawley rats, the second-order reaction rate constants were determined for the reaction of SO with cysteine (through both the alpha- and beta-carbons of SO), N-terminal valine (through the beta-carbon of SO), and carboxylic acid (presumably through both the alpha- and beta-carbons of SO) residues in Hb. The rate constants for cysteine adducts vary dramatically between species [2.04, 10.7, 133 L (mol Hb)-1 h-1 (alpha binding) for humans, mice and rats, respectively] and [0.078, 2.16, 20.4 L (mol Hb)-1 h-1 (beta binding), respectively]. The considerably higher rate of reaction with cysteine in rat Hb probably reflects the presence of an additional cysteine residue at position beta 125. Although the rate constants for valine adducts (1.82, 0.80, 0.29 L (mol Hb)-1 h-1, respectively) and COOH adducts (3.55, 1.94, 2.37 L (mol Hb)-1 h-1, respectively) are much more consistent, the inter-species differences are statistically significant for the reaction of SO with the N-terminal valine of Hb. Following the i.p. administration of styrene to mice and styrene and SO to rats, the levels of adducts at each of these sites were used in conjunction with the calculated rate constants to predict the integrated blood doses of SO. While the SO doses predicted from cysteine and valine adducts were very similar, that based upon COOH-binding was significantly different, presumably due to the instability of SO-COOH adducts. This research affirms the use of both cysteine and valine adducts, but not carboxylic acid adducts, as biomarkers of exposure to styrene and SO.