Species variations in the metabolism of liposoluble organochlorine compounds by hepatic microsomal monooxygenase: comparative kinetics in four vertebrate species

Review of existing terrestrial bioaccumulation models and terrestrial bioaccumulation modeling needs for organic chemicals

Protocols for terrestrial bioaccumulation assessments are far less-developed than for aquatic systems. This article reviews modeling approaches that can be used to assess the terrestrial bioaccumulation potential of commercial organic chemicals. Models exist for plant, invertebrate, mammal, and avian species and for entire terrestrial food webs, including some that consider spatial factors. Limitations and gaps in terrestrial bioaccumulation modeling include the lack of QSARs for biotransformation and dietary assimilation efficiencies for terrestrial species; the lack of models and QSARs for important terrestrial species such as insects, amphibians and reptiles; the lack of standardized testing protocols for plants with limited development of plant models; and the limited chemical domain of existing bioaccumulation models and QSARs (e.g., primarily applicable to nonionic organic chemicals). There is an urgent need for high-quality field data sets for validating models and assessing their performance. There is a need to improve coordination among laboratory, field, and modeling efforts on bioaccumulative substances in order to improve the state of the science for challenging substances.

A compilation of in vitro rate and affinity values for xenobiotic biotransformation in fish, measured under physiological conditions

Scientific literature from the past 25 years was searched to obtain in vitro biotransformation rate and affinity data for fish. To maximize the environmental relevance of this dataset, we focused on studies conducted at multiple substrate concentrations, and established acceptance criteria with respect to assay temperature and pH. Altogether, enzyme rate and affinity parameters are provided for 43 species and 77 compounds. In all but three instances, the reported reactions exhibited saturation at high substrate concentrations and could be used to calculate Michaelis-Menten rate (Vmax) and affinity (Km) constants. Most of this information was obtained using in vitro systems derived from liver tissue. Information from non-hepatic tissues was included, however, to provide a basis for comparisons among tissues. Where possible, in vitro enzyme parameters were examined to compare: (1) hepatic metabolism of a common substrate within a species, (2) hepatic metabolism of common substrates by different species, and (3) metabolism of a common substrate by different tissues of one species. Comparisons within species highlight a number of factors that may substantially influence xenobiotic metabolism in fish including gender, life stage, and acclimation temperature. Limited data suggest that Vmax and Km for the same reaction may vary by up to three orders of magnitude among species.

Determination of xenobiotic intrinsic clearance in freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat and its application in bioaccumulation assessment

Bioaccumulation in fish depends on the dynamics of various processes that involve fish uptake, storage, and elimination of xenobiotics. Elimination via fish biotransformation is a primary process that can be evaluated in an in vitro system to improve the performance of the prediction of xenobiotic bioaccumulation potentials. In this study, values of intrinsic clearance (CLint) of seven reference compounds (atrazine, molinate, 4,4-bis(dimethylamino)-benzophenone, 4-nonylphenol, 2,4-di-tert-butylphenol, trifluralin, benzo(a)pyrene) in hepatocytes freshly isolated from rainbow trout and rat were determined using a substrate depletion approach. Atrazine was metabolized in rat hepatocytes with a CLint value of 3.81 +/- 1.96 mL/h/ 10(6) cells, whereas in trout hepatocytes, the clearance was not significant until very high cell concentration was used and the rate was estimated to be approximately 0.002 mL/h/10(6) cells. Intrinsic clearance values for all other compounds were 5.5-78.5-fold lower in trout hepatocytes than those in rat hepatocytes. Trout hepatic clearance (CL(H)) values were extrapolated from the CLint values using a "well-stirred" liver model. Biotransformation rate constants (kMET) of the compounds in trout were subsequently estimated and used as inputs to a kinetic model for the prediction of bioconcentration factors (BCF) in fish. Compared to the BCF values predicted without consideration of fish biotransformation, the inclusion of estimated kMET values significantly improved fish BCF predictions for the reference compounds. This study demonstrates a framework for future bioaccumulation assessment of xenobiotics using combined information of the physical-chemical properties of the compounds and the biotransformation potentials of the compounds in fish.

Comparison of basal level metabolic enzyme activities of freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat

Biotransformation plays a key role in detoxification, bioactivation and bioaccumulation of xenobiotics in fish. Biotransformation capabilities in fish, however, are not as thoroughly characterized as in mammals. In this study, basal level activities of 7-ethoxyresorufin O-dealkylase (EROD), 7-methoxyresorufin O-dealkylase (MROD), 7-pentoxyresorufin O-dealkylase, chlorzoxazone 6-hydroxylase, testosterone 6beta-hydroxylase, lauric acid 11-hydroxylase, and glutathione S-transferase in freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat were obtained and compared. All the activities, when normalized to cellular protein concentrations, were significantly lower in rainbow trout hepatocytes than those in rat hepatocytes. Rainbow trout cytochrome P450 (CYP) 2B-, 2E1-, and 3A-like activities were respectively 14-, 18.4-, and 11.8-fold lower than rat. The smallest difference between the two species was CYP1A-type activities (EROD and MROD, 3.4- and 3.7-fold lower in rainbow trout, respectively). Our results suggest that the relative importance of CYP1A enzymes (among CYP subfamilies) in the biotransformation of xenobiotics in rainbow trout is likely much greater than that in mammals.

Intestinal capacity of P-glycoprotein is higher in the juniper specialist, Neotoma stephensi, than the sympatric generalist, Neotoma albigula

Permeability-glycoprotein (Pgp) is a membrane-bound, ATP-dependent, transport protein that excludes many cytotoxic compounds including plant metabolites and pollutants from the barrier epithelia of many tissues including the small intestine. We hypothesized that intestinal Pgp capacity would be higher in Neotoma stephensi, a specialist on Juniperus monosperma known to be high in plant toxins, than the sympatric generalist, Neotoma albigula, which consumes juniper in the field, but is unable to tolerate a high juniper diet. We measured Pgp activity as the difference in accumulation of a known Pgp substrate, digoxin, between everted sections of small intestine exposed to ethanol vehicle control and a maximal level of a known competitive inhibitor of Pgp, cyclosporin A. We estimated intestinal capacity by averaging Pgp activity along the intestine and multiplying by total small intestine mass. These first measures of Pgp in wild mammals show a significant difference among species with the juniper specialist, N. stephensi, exhibiting a 2.4 fold higher capacity than the generalist, N. albigula. This result suggests that Pgp may play a role in the ability of N. stephensi to tolerate juniper.

Ranking terrestrial vertebrate species for utility in biomonitoring and vulnerability to environmental contaminants

The measurement of contaminant tissue concentrations or exposure-related effects in biota has been used extensively to monitor pollution and environmental health. Terrestrial vertebrates have historically been an important group of species in such evaluations, not only because many are excellent sentinels of environmental contamination, but also because they are valued natural resources in their own right that may be adversely affected by toxicant exposure. Selection of appropriate vertebrates for biomonitoring studies frequently relies on expert opinion, although a few rigorous schemes are in use for predicting vulnerability of birds to the adverse effects of petroleum crude oil. A Utility Index that ranks terrestrial vertebrate species as potential sentinels of contaminants in a region, and a Vulnerability Index that assesses the threat of specific groups of contaminants to these species, have been developed to assist decision makers in risk assessments of persistent organic pollutants, cholinesterase-inhibiting pesticides, petroleum crude oil, mercury, and lead shot. Twenty-five terrestrial vertebrate species commonly found in Atlantic Coast estuarine habitat (Rattner et al. 2001a) were ranked for their utility as biomonitors of contamination and their vulnerability to pollutants in this region. No single species, taxa, or class of vertebrates was found to be an ideal sentinel for all groups of contaminants. Although birds have overwhelmingly been used to monitor contaminants compared to other terrestrial vertebrate classes, the nonmigratory nature and dietary habits of the snapping turtle and mink consistently resulted in ranking these species as excellent sentinels as well. Vulnerability of Atlantic Coast populations of these species varied considerably among groups of contaminants. Usually a particular species was found to be at high risk to only one or two groups of contaminants, although a noteworthy exception is the bald eagle, which is highly vulnerable to all five of the contaminant groups examined. This index could be further enhanced by generation of additional comparative toxicity data to facilitate interspecific extrapolations. The Utility and Vulnerability Indices have application to many types of habitats in addition to estuaries and are of value to natural resource and risk managers that routinely conduct local, regional, or national environmental quality assessments.