In vitro kinetics of styrene and styrene oxide metabolism in rat, mouse, and human

Abstract

Styrene oxide (SO), a labile metabolite of styrene, is generally accepted as being responsible for any genotoxicity associated with styrene. To better define the hazard associated with styrene, the activity of the enzymes involved in the formation (monooxygenase) and destruction of SO (epoxide hydrolase and glutathione-S-transferase) were measured in the liver and lungs from naive and styrene-exposed male Sprague-Dawley rats and B6C3F1 mice (three daily 6-h inhalation exposures at up to 600 ppm styrene) and Fischer 344 rats (four daily 6-h inhalation exposures at up to 1000 ppm styrene), and in samples of human liver tissue. Additionally, the time course of styrene and SO in the blood was measured following oral administration of 500 mg styrene/kg body weight to naive Fischer rats and rats previously exposed to 1000 ppm styrene. The affinity of hepatic monooxygenase for styrene, as measured by the Michaelis constant (Km), was similar in the rat, mouse, and human. Based on the Vmax for monooxygenase activity and the relative liver and body size, the mouse had the greatest capacity and humans the lowest capacity to form SO from styrene. In contrast, human epoxide hydrolase and a greater affinity (i.e., lower Km) for SO than epoxide hydrolase from rats or mice while the apparent Vmax for epoxide hydrolase was similar in the rat, mouse, and human liver. However, the activity of epoxide hydrolase relative to monooxygenase activity was much greater in the human than in the rodent liver. Hepatic glutathione-S-transferase activity, as indicated by the Vmax, was 6- to 33-fold higher than epoxide hydrolase activity. However, the significance of the high glutathione-S-transferase activity is unknown because hydrolysis, rather than conjugation, is the primary pathway for SO detoxification in vivo. Human hepatic glutathione-S-transferase activity was extremely variable between individual human livers and much lower than in rat or mouse liver. Prior exposure to styrene had no effect on monooxygenase activity or on blood styrene levels in rats given a large oral dose of styrene. In contrast, prior exposure to styrene increased hepatic epoxide hydrolase activity 1.6-fold and resulted in lower (0.1 > P > 0.05) blood SO levels in rats given a large oral dose of styrene. Qualitatively, these data indicate that the mouse has the greatest capacity and the human the lowest capacity to form SO. In addition, human liver should be more effective than rodent liver in hydrolyzing low levels of SO.(ABSTRACT TRUNCATED AT 400 WORDS)

CYP2E1 Metabolism of Styrene Involves Allostery

“…sible through a reciprocal Lineweaver-Burk plot of the data, which unevenly weighs data points and masks non-Michaelis-Menten behavior. For comparison to previous studies (Mendrala et al, 1993;Nakajima et al, 1994;Wenker et al, 2001), we recast data from our microsomal reactions as a Lineweaver-Burk plot shown in Fig. 8.…”

“…This degradative pathway occurs very slowly in water but is rapidly catalyzed by epoxide hydrolase. CYP2E1 is primarily responsible for the initial critical step in styrene activation to a reactive metabolite (Mendrala et al, 1993;Nakajima et al, 1994;Kim et al, 1997;Wenker et al, 2001), and thus, an understanding of the contribution of CYP2E1 to the metabolic clearance of styrene is necessary for adequately estimating risk associated with exposure to this pollutant.…”

“…The highaffinity activity was attributed to CYP2E1 on the basis of correlative activity assays (Kim et al, 1997;Wenker et al, 2001) and inhibitor phenotyping (Koop, 1990;Wenker et al, 2001). Kinetic data were analyzed by the Michaelis-Menten equation to obtain V max and K m parameters used for modeling metabolic clearance (V max /K m ) of the pollutant (Mendrala et al, 1993;Nakajima et al, 1994;Wenker et al, 2001). This strategy demonstrated that a high-affinity (CYP2E1) activity was approximately 100-fold more efficient than a low-affinity activity, indicating that the metabolic efficiency of CYP2E1 dominates styrene metabolism (Wenker et al, 2001).…”

Detection of Styrene and Styrene Oxide‐Induced DNA Damage in Various Organs of Mice Using the Comet Assay

“…A similar type of dose-response curve was also reported by Osterman-Golkar et al (1999, investigat-ing the adduct formation in haemoglobin of mice exposed to these compounds. This sublinearity can indicate the saturation of styrene oxide detoxification at higher doses, possibly as a result of the saturable activity of glutathione transferase (Mendrala et al 1993). It also appears that the further metabolism of styrene oxide is saturable.…”

Urinary mandelic acid and hemoglobin adducts in fiberglass-reinforced plastics workers exposed to styrene.

“…The ep oxide is detoxified by epoxide hydrol ase. Mendrala et al (15) reported that the ac tivity of the epoxi de hyd rolase relative to mon oo xygenase ac tivity was much greater in hum an liver than in mou se liver. Th e apparent half-time for styrene-7,8-oxi de in mouse and human wa s calculated to be 38 and 1.8 min, resp ectivel y.…”