The bioavailability of polycyclic aromatic hydrocarbons from different dose media after single and sub-chronic exposure in juvenile swine

Using soil enzyme Vmax as an indicator to evaluate the ecotoxicity of lower-ring polycyclic aromatic hydrocarbons in soil: Evidence from fluorescein diacetate hydrolase kinetics

Fluorescein diacetate hydrolase (FDA hydrolase) is a reliable biochemical biomarker of changes in soil microbial activity and quality. However, the effect and mechanism of lower-ring polycyclic aromatic hydrocarbons (PAHs) on soil FDA hydrolase are still unclear. In this work, we investigated the effects of two typical lower-ring PAHs, naphthalene (Nap) and anthracene (Ant), on the activity and kinetic characteristics of FDA hydrolases in six soils differing in their properties. Results demonstrated that the two PAHs severely inhibited the activities of the FDA hydrolase. The values of V_max and K_m dropped by 28.72-81.24 % and 35.84-74.47 % at the highest dose of Nap, respectively, indicating an uncompetitive inhibitory mechanism. Under Ant stress, the values of V_max decreased by 38.25-84.99 %, and the K_m exhibited two forms, unchanged and decreased (74.00-91.61 %), indicating uncompetitive and noncompetitive inhibition. The inhibition constant (K_i) of the Nap and Ant ranged from 0.192 to 1.051 and 0.018 to 0.087 mM, respectively. The lower K_i of Ant compared to Nap indicated a higher affinity for enzyme-substrate complex, resulting in higher toxicity of Ant than Nap to soil FDA hydrolase. The inhibitory effect of Nap and Ant on soil FDA hydrolase was mainly affected by soil organic matter (SOM). SOM influenced the affinity of PAHs with enzyme-substrate complex, which resulted in a difference in PAHs toxicity to soil FDA hydrolase. The enzyme kinetic V_max was a more sensitive indicator than enzyme activity to evaluate the ecological risk of PAHs. This research offers a strong theoretical foundation for quality control and risk evaluation of PAH-contaminated soils through a soil enzyme-based approach.

Is received dose from ingested soil independent of soil PAH concentrations?-Animal model results

Polycyclic aromatic hydrocarbon (PAH) bioavailability from ingested soils will vary between soils; however, the nature of this variation is not well characterized. A juvenile swine model was used to link external exposure to internal benzo[a]pyrene (BaP) and anthracene exposure following oral PAH ingestion of 27 different impacted site soils, soots, or spiked artificial soils. Internal exposure of BaP and anthracene, represented by area under the plasma-time curve, did not relate to soil concentration in impacted site soils, but did relate in spiked artificial soil. Point of departure modeling identified soil PAH concentrations greater than 1900 mg kg(-1) as the point where area under the curve becomes proportional to external dose. A BaP internal exposure below 1900 mg kg(-1) had an upper 95% confidence interval estimate of 33% of external exposure. Weak relationships between soil:simulated gastrointestinal fluid PAH partitioning and area under the curve values suggest that differences in internal PAH exposure between soils may not be dominated by differences in PAH partitioning. The data seem to best support exposure assessment assuming constant internal PAH exposure below soil concentrations of 1900 mg kg(-1) . However, because constant internal exposure would challenge several existing paradigms, a bioavailability estimate of 33% of the external exposure is suggested as a likely workable solution. Environ Toxicol Chem 2016;35:2261-2269. © 2016 SETAC.

Comparison of oral bioavailability of benzo[a]pyrene in soils using rat and swine and the implications for human health risk assessment

BACKGROUND

There are many uncertainties concerning variations in benzo[a]pyrene (B[a]P) soil guidelines protecting human health based on carcinogenic data obtained in animal studies. Although swine is recognised as being much more representative of the human child in terms of body size, gut physiology and genetic profile the rat/mice model is commonly used in practice.

OBJECTIVES

We compare B[a]P bioavailability using a rat model to that estimated in a swine model, to investigate the correlation between these two animal models. This may help reduce uncertainty in applying bioavailability to human health risk assessment.

METHODS

Twelve spiked soil samples and a spiked silica sand (reference material) were dosed to rats in parallel with a swine study. B[a]P bioavailability was estimated by the area under the plasma B[a]P concentration-time curve (AUC) and faecal excretion as well in the rats. Direct comparison between the two animal models was made for: firstly, relative bioavailability (RB) using AUC assay; and secondly, the two assays in the rat model.

RESULTS

Both AUC and faecal excretion assays showed linear dose-response for the reference material. However, absolute bioavailability was significantly higher when using faecal excretion assay (p<0.001). In aged soils faecal excretion estimated based on solvent extraction was not accurate due to the form of non-extractable fraction through ageing. A significant correlation existed between the two models using RB for soil samples (RBrat=0.26RBswine+17.3, R(2)=0.70, p<0.001), despite the regression slope coefficient revealing that the rat model would underestimate RB by about one quarter compared to using swine.

CONCLUSIONS

In the comparison employed in this study, an interspecies difference of four in RB using AUC assay was identified between the rat and swine models regarding pharmacokinetic differences, which supported the body weight scaling method recommended by US EPA. Future research should focus on the carcinogenic competency (pharmacodynamics) used in experiment animals and humans.

Predicting Polycyclic Aromatic Hydrocarbon Bioavailability to Mammals from Incidentally Ingested Soils Using Partitioning and Fugacity

Soil and dust ingestion is one of the major human exposure pathways to contaminated soil; however, pollutant transfer from ingested substances to humans cannot currently be confidently predicted. Soil polycyclic aromatic hydrocarbon (PAH) bioavailability is likely dependent upon properties linked to chemical potential and partitioning such as fugacity, fugacity capacity, soil organic carbon, and partitioning to simulated intestinal fluids. We estimated the oral PAH bioavailability of 19 historically contaminated soils fed to juvenile swine. Between soils, PAH blood content, with the exception of benzo(a)pyrene, was not linked to fugacity. In contrast, between individual PAHs, using partitioning explained PAH blood content (area under the curve = 0.47 log fugacity + 0.34, r(2) = 0.68, p < 0.005, n = 14). Soil fugacity capacity predicts PAH soil concentration with an average slope of 0.30 (μg PAH g(-1) soil) Pa(-1) and r(2)'s of 0.61-0.73. Because PAH blood content was independent of soil concentration, soil fugacity correlated to PAH bioavailability via soil fugacity's link to soil concentration. In conclusion, we can use fugacity to explain PAH uptake from a soil into blood. However, something other than partitioning is critical to explain the differences in PAH uptake into blood between soils.