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In Vitrotoin VivoExtrapolation for Trichloroethylene Metabolism in Humans

https://doi.org/10.1006/taap.1998.8485Get rights and content

Abstract

The useof in vitrosystems in the assessment of xenobiotic metabolism has distinct advantages and disadvantages. While isolated hepatocytes and microsomes prepared from human liver may be used to generate data for comparisons among species andin vitrosystems, such comparisons are generally performed on the basis of microsomal protein or million (viable) hepatocytes. Recently,in vitrodata have been investigated for their value as quantitative predictors ofin vivometabolic capacity. Because of the existence of large amounts of trichloroethylene (TRI) data in the human, we have examined the metabolism of TRI as a case study in the development of a method to compare metabolism across species usingin vitrosystems and for extrapolation of metabolic rates fromin vitrotoin vivo.TRI is well metabolized by human hepatocytes in culture with aKmof 266 ± 202 ppm (mean ± SD) in headspace and aVmaxof 16.1 ± 12.9 nmol/h/106viable hepatocytes. We determined that human liver contains approximately 116 × 106hepatocytes and 20.8 mg microsomal protein/g, based on DNA recovery and glucose-6-phosphatase activity, respectively. Thus, the microsomal protein content of hepatocytes is 179 μg microsomal protein/106isolated hepatocytes. The microsomal apparentVmaxvalue of 1589 pmol/min/mg microsomal protein extrapolates to 17.07 nmol/h/106hepatocytes. The combination of protein recovery and metabolic rate predicted aVmaxof approximately 1400 nmol/h/g human liver, which, when extrapolated and incorporated into an existing physiologically based pharmacokinetic (PBPK) model for TRI, slightly underpredicted TRI metabolism in the intact human. The quantitation, extrapolation, and inclusion of extrahepatic and cytochrome P450 (CYP)-independent TRI metabolism may increase the predictive value of this approach.

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      Furthermore, Reynaud et al. (2012) observed an increase in bioconcentration factor for FIP over six days in green frogs, which was the opposite of the current study. Extrapolation from Vmax in vitro liver microsomes to Vmax in vivo for risk assessments was similar to the method in Lipscomb et al. (1998) (Table 2). DIA and DEA had the lowest values at 0.47 and 0.46 mg h−1 kg−1, respectively, while the highest value was for TDN at 2.38 mg h−1 kg−1.

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    A. M. Goldberg

    1

    To whom correspondence should be addressed at U.S. Environmental Protection Agency, National Center for Environmental Assessment, 26 West Martin Luther King Jr Drive, MC-190, Cincinnati, Ohio 45268. Fax: (513) 569–7916.

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