Wildfire, Smoke Exposure, Human Health, and Environmental Justice Need to be Integrated into Forest Restoration and Management

PURPOSE OF REVIEW

Increasing wildfire size and severity across the western United States has created an environmental and social crisis that must be approached from a transdisciplinary perspective. Climate change and more than a century of fire exclusion and wildfire suppression have led to contemporary wildfires with more severe environmental impacts and human smoke exposure. Wildfires increase smoke exposure for broad swaths of the US population, though outdoor workers and socially disadvantaged groups with limited adaptive capacity can be disproportionally exposed. Exposure to wildfire smoke is associated with a range of health impacts in children and adults, including exacerbation of existing respiratory diseases such as asthma and chronic obstructive pulmonary disease, worse birth outcomes, and cardiovascular events. Seasonally dry forests in Washington, Oregon, and California can benefit from ecological restoration as a way to adapt forests to climate change and reduce smoke impacts on affected communities.

RECENT FINDINGS

Each wildfire season, large smoke events, and their adverse impacts on human health receive considerable attention from both the public and policymakers. The severity of recent wildfire seasons has state and federal governments outlining budgets and prioritizing policies to combat the worsening crisis. This surging attention provides an opportunity to outline the actions needed now to advance research and practice on conservation, economic, environmental justice, and public health interests, as well as the trade-offs that must be considered. Scientists, planners, foresters and fire managers, fire safety, air quality, and public health practitioners must collaboratively work together. This article is the result of a series of transdisciplinary conversations to find common ground and subsequently provide a holistic view of how forest and fire management intersect with human health through the impacts of smoke and articulate the need for an integrated approach to both planning and practice.

Adapting western North American forests to climate change and wildfires: 10 common questions

We review science-based adaptation strategies for western North American (wNA) forests that include restoring active fire regimes and fostering resilient structure and composition of forested landscapes. As part of the review, we address common questions associated with climate adaptation and realignment treatments that run counter to a broad consensus in the literature. These include the following: (1) Are the effects of fire exclusion overstated? If so, are treatments unwarranted and even counterproductive? (2) Is forest thinning alone sufficient to mitigate wildfire hazard? (3) Can forest thinning and prescribed burning solve the problem? (4) Should active forest management, including forest thinning, be concentrated in the wildland urban interface (WUI)? (5) Can wildfires on their own do the work of fuel treatments? (6) Is the primary objective of fuel reduction treatments to assist in future firefighting response and containment? (7) Do fuel treatments work under extreme fire weather? (8) Is the scale of the problem too great? Can we ever catch up? (9) Will planting more trees mitigate climate change in wNA forests? And (10) is post-fire management needed or even ecologically justified? Based on our review of the scientific evidence, a range of proactive management actions are justified and necessary to keep pace with changing climatic and wildfire regimes and declining forest heterogeneity after severe wildfires. Science-based adaptation options include the use of managed wildfire, prescribed burning, and coupled mechanical thinning and prescribed burning as is consistent with land management allocations and forest conditions. Although some current models of fire management in wNA are averse to short-term risks and uncertainties, the long-term environmental, social, and cultural consequences of wildfire management primarily grounded in fire suppression are well documented, highlighting an urgency to invest in intentional forest management and restoration of active fire regimes.

Mixed-conifer forest reference conditions for privately owned timberland in the southern Cascade Range

The overwhelming majority of information on historical forest conditions in western North America comes from public lands, which may provide an incomplete description of historical landscapes. In this study we made use of an archive containing extensive timber survey data collected in the early 1920s from privately owned forestland. These data covered over 50,000 ha and effectively represent a 19% sample of the entire area. The historical forest conditions reconstructed from these data fit the classic model of frequent-fire forests: large trees, low density, and pine-dominated. However, unlike other large-scale forest reconstructions, our study area exhibited relatively low overall variability in forest structure and composition across the historical landscape. Despite having low variability, our analyses revealed evidence of biophysical controls on tree density and pine fraction. Annual climatic variables most strongly explained the range in historical tree densities, whereas historical pine fraction was explained by a combination of topographic and climatic variables. Contemporary forest inventory data collected from both public and private lands within the same general area, albeit not a direct remeasurement, revealed substantial increases in tree density and greatly reduced pine fractions relative to historical conditions. Contemporary forests exhibited a far greater range in these conditions than what existed historically. These findings suggest that private forestland managed with multiaged silviculture may be similar to public forestland with respect to departure in forest structure and compositions from that of historical forests. However, there may be differences between management objectives that favor timber production, more typical on private lands, vs. those that favor restoration, increasingly supported on public lands.

Forest Restoration and Fuels Reduction: Convergent or Divergent?

For over 20 years, forest fuel reduction has been the dominant management action in western US forests. These same actions have also been associated with the restoration of highly altered frequent-fire forests. Perhaps the vital element in the compatibility of these treatments is that both need to incorporate the salient characteristics that frequent fire produced—variability in vegetation structure and composition across landscapes and the inability to support large patches of high-severity fire. These characteristics can be achieved with both fire and mechanical treatments. The possible key to convergence of fuel reduction and forest restoration strategies is integrated planning that permits treatment design flexibility and a longer-term focus on fire reintroduction for maintenance. With changing climate conditions, long-term forest conservation will probably need to be focused on keeping tree density low enough (i.e., in the lower range of historic variation) for forest conditions to adapt to emerging disturbance patterns and novel ecological processes.

Extreme mid-winter drought weakens tree hydraulic-carbohydrate systems and slows growth

Rising temperatures and extended periods of drought compromise tree hydraulic and carbohydrate systems, threatening forest health globally. Despite winter's biological significance to many forests, the effects of warmer and dryer winters on tree hydraulic and carbohydrate status have largely been overlooked. Here we report a sharp and previously unknown decline in stem water content of three conifer species during California's anomalous 2015 mid-winter drought that was followed by dampened spring starch accumulation. Recent precipitation and seasonal vapor pressure deficit (VPD) anomaly, not absolute VPD, best predicted the hydraulic patterns observed. By linking relative water content and hydraulic conductivity (K_h ), we estimated that stand-level K_h declined by 52% during California's 2015 mid-winter drought, followed by a 50% reduction in spring starch accumulation. Further examination of tree increment records indicated a concurrent decline of growth with rising mid-winter, but not summer, VPD anomaly. Thus, our findings suggest a seasonality to tree hydraulic and carbohydrate declines, with consequences for annual growth rates, raising novel physiological and ecological questions about how rising winter temperatures will affect forest vitality as climate changes.

Scale-dependent effects of post-fire canopy cover on snowpack depth in montane coniferous forests

Winter snowpack in dry montane regions provides a valuable ecosystem service by storing water into the growing season. Wildfire in coniferous montane forests has the potential to indirectly affect snowpack accumulation and ablation (mass loss) rates by reducing canopy cover, which reduces canopy interception of snow but also increases solar radiation and wind speed. These counteracting effects create uncertainty regarding the canopy conditions that maximize post-fire snowpack duration, which is of concern as montane regions across the western United States experience increasingly warm, dry winters with below-average snowpack. The net effect of wildfire on snowpack depth and duration across the landscape is uncertain, and likely scale dependent. In this study, I tested whether intermediate levels of wildfire severity maximize snowpack depth by increasing accumulation while slowing ablation, using gridded, repeated snow depth measurements from three fires in the Sierra Nevada of California. Increasing fire severity had a strong negative effect on snowpack depth, suggesting that increased ablation after fire, rather than increased accumulation, was the dominant control over snowpack duration. Contrary to expectations, the unburned forest condition had the highest overall snowpack depth, and mean snow depth among all site visits was reduced by 78% from unburned forest to high-severity fire. However, at the individual tree scale, snowpack depth was greater under canopy openings than underneath canopy, controlling for effects of fire severity and aspect. This apparent paradox in snowpack response to fire at the stand vs. individual tree scales is likely due to greater variation in canopy cover within unburned and very low severity areas, which creates smaller areas for snow accumulation while reducing ablation via shading. Management efforts to maximize snowpack duration in montane forests should focus on retaining fine-scale heterogeneity in forest structure.

Average Stand Age from Forest Inventory Plots Does Not Describe Historical Fire Regimes in Ponderosa Pine and Mixed-Conifer Forests of Western North America

Quantifying historical fire regimes provides important information for managing contemporary forests. Historical fire frequency and severity can be estimated using several methods; each method has strengths and weaknesses and presents challenges for interpretation and verification. Recent efforts to quantify the timing of historical high-severity fire events in forests of western North America have assumed that the “stand age” variable from the US Forest Service Forest Inventory and Analysis (FIA) program reflects the timing of historical high-severity (i.e. stand-replacing) fire in ponderosa pine and mixed-conifer forests. To test this assumption, we re-analyze the dataset used in a previous analysis, and compare information from fire history records with information from co-located FIA plots. We demonstrate that 1) the FIA stand age variable does not reflect the large range of individual tree ages in the FIA plots: older trees comprised more than 10% of pre-stand age basal area in 58% of plots analyzed and more than 30% of pre-stand age basal area in 32% of plots, and 2) recruitment events are not necessarily related to high-severity fire occurrence. Because the FIA stand age variable is estimated from a sample of tree ages within the tree size class containing a plurality of canopy trees in the plot, it does not necessarily include the oldest trees, especially in uneven-aged stands. Thus, the FIA stand age variable does not indicate whether the trees in the predominant size class established in response to severe fire, or established during the absence of fire. FIA stand age was not designed to measure the time since a stand-replacing disturbance. Quantification of historical “mixed-severity” fire regimes must be explicit about the spatial scale of high-severity fire effects, which is not possible using FIA stand age data.

Snowpack, fire, and forest disturbance: interactions affect montane invasions by non-native shrubs

Montane regions worldwide have experienced relatively low plant invasion rates, a trend attributed to increased climatic severity, low rates of disturbance, and reduced propagule pressure relative to lowlands. Manipulative experiments at elevations above the invasive range of non-native species can clarify the relative contributions of these mechanisms to montane invasion resistance, yet such experiments are rare. Furthermore, global climate change and land use changes are expected to cause decreases in snowpack and increases in disturbance by fire and forest thinning in montane forests. We examined the importance of these factors in limiting montane invasions using a field transplant experiment above the invasive range of two non-native lowland shrubs, Scotch broom (Cytisus scoparius) and Spanish broom (Spartium junceum), in the rain-snow transition zone of the Sierra Nevada of California. We tested the effects of canopy closure, prescribed fire, and winter snow depth on demographic transitions of each species. Establishment of both species was most likely at intermediate levels of canopy disturbance, but at this intermediate canopy level, snow depth had negative effects on winter survival of seedlings. We used matrix population models to show that an 86% reduction in winter snowfall would cause a 2.8-fold increase in population growth rates in Scotch broom and a 3.5-fold increase in Spanish broom. Fall prescribed fire increased germination rates, but decreased overall population growth rates by reducing plant survival. However, at longer fire return intervals, population recovery between fires is likely to keep growth rates high, especially under low snowpack conditions. Many treatment combinations had positive growth rates despite being above the current invasive range, indicating that propagule pressure, disturbance, and climate can all strongly affect plant invasions in montane regions. We conclude that projected reductions in winter snowpack and increases in forest disturbance are likely to increase the risk of invasion from lower elevations.

Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations

Plant communities in abiotically stressful, or 'harsh', habitats have been reported to be less invaded by non-native species than those in more moderate habitats. Here, we synthesize descriptive and experimental evidence for low levels of invasion in habitats characterized by a variety of environmental stressors: low nitrogen; low phosphorus; saline, sodic or alkaline soils; serpentine soils; low soil moisture; shallow/rocky soils; temporary inundation; high shade; high elevation; and high latitude. We then discuss major categories of hypotheses to explain this pattern: the propagule limitation mechanism suggests invasion of harsh sites is limited by relatively low arrival rates of propagules compared with more moderate habitats, while invasion resistance mechanisms suggest that harsh habitats are inherently less invasible due to stressful abiotic conditions and/or increased effects of biotic resistance from resident organisms. Both propagule limitation and invasion resistance may simultaneously contribute to low invadedness of harsh sites, but the management implications of these mechanisms differ. If propagule limitation is more important, managers should focus on reducing the likelihood of propagule introductions. If invasion resistance mechanisms are in play, managers should focus on restoring or maintaining harsh conditions at a site to reduce invasibility.

Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper (Schinus terebinthifolius)

* Fire disturbance can mediate the invasion of ecological communities by nonnative species. Nonnative plants that modify existing fire regimes may initiate a positive feedback that can facilitate their continued invasion. Fire-sensitive plants may successfully invade pyrogenic landscapes if they can inhibit fire in the landscape. * Here, we investigated whether the invasive shrub Brazilian pepper (Schinus terebinthifolius) can initiate a fire-suppression feedback in a fire-dependent pine savanna ecosystem in the southeastern USA. * We found that prescribed burns caused significant (30-45%) mortality of Brazilian pepper at low densities and that savannas with more frequent fires contained less Brazilian pepper. However, high densities of Brazilian pepper reduced fire temperature by up to 200 degrees C, and experienced as much as 80% lower mortality. * A cellular automaton model was used to demonstrate that frequent fire may control low-density populations, but that Brazilian pepper may reach a sufficient density during fire-free periods to initiate a positive feedback that reduces the frequency of fire and converts the savanna to an invasive-dominated forest.

Transendothelial permeability of chlorpyrifos in RBE4 monolayers is modulated by astrocyte-conditioned medium

The immortalized rat brain endothelium 4 (RBE4) cell line preserves many features of the in vivo brain endothelium. It has been used as an in vitro model of the blood-brain barrier (BBB). Astrocyte-endothelial cell interactions are crucial for maintenance of BBB characteristics. The present study investigated morphological and permeability properties of the RBE4 cell line. Immunohistochemical studies showed positive staining in RBE4 cells for E-cadherin, a Ca(2+)-dependent cell-cell adhesion molecule. Western blot immunoassay showed that RBE4 cells consistently express E-cadherin and that its expression significantly increased (P<0.001) in the presence of astrocyte-conditioned medium (ACM). The transendothelial permeability of chlorpyrifos, an organophosphorus insecticide, was significantly decreased (P<0.001) when the RBE4 cells were grown in ACM compared with control medium. Additional studies were carried out to determine whether chlorpyrifos is a substrate for the multidrug resistance protein, P-glycoprotein (P-gp). No significant change in chlorpyrifos transendothelial permeability was noted in the presence of verapamil, a P-gp blocker. Thus, in this system, chlorpyrifos is not a substrate for P-gp. This work demonstrates that with additional refinements the RBE4 monolayers might serve as a useful in vitro model for the study of BBB permeability and modulation by astrocyte-derived soluble factors.

The mammary tumor response in triazine-treated female rats: a threshold-mediated interaction with strain and species-specific reproductive senescence

Triazine herbicides are among the most heavily used agricultural pesticides. Although they possess a very low acute toxicity in animals, a mammary tumor response has been consistently observed in Sprague-Dawley (SD) female rats following chronic oral dosing of atrazine and simazine at and above maximum tolerated doses. However, a substantial collection of detailed research has clearly shown that triazines are not genotoxic or mutagenic, nor do they possess estrogenic agonist activity that might promote mammary tumor growth. Examination of estrous cycling records of atrazine-treated SD rats revealed a premature appearance of persistent estrous episodes, beyond the prevalent occurrence normally seen in untreated, aging SD rats. A significant correlation has been found between early or severe estrous cycle disruption of atrazine-treated rats and the early appearance of mammary tumors. In studies using SD female rats fed atrazine for 6 months, then ovariectomized and administered an estrogen-containing silastic s.c. implant, a deficient luteinizing hormone surge was observed at a 400 parts per million (ppm) dose, but not at 25 or 50 ppm. Because SD rats exhibiting persistent estrus also have a prolonged elevation of estrogen secretion, it is proposed that the triazine-associated mammary tumor response is promoted by the test animal's own estrogen from ovarian follicles that fail to ovulate because gonadotropin surge sufficiency is blocked by the high dose of herbicide. It is further proposed that, because reproductive senescence in SD rats is fundamentally different from menopause in women, the animal response to dosing, as well as the enormous requisite dosing level, establishes a safety margin of very low risk to human health from this mode of action.

A risk characterization for atrazine: oncogenicity profile

An extensive safety database has been developed for the chlorotriazine herbicide, atrazine. The results from five oncogenicity studies conducted in the Sprague-Dawley rat, two studies in the Fischer 344 rat, and two studies in the CD-1 mouse were reviewed. No increase in the incidence of tumors of any type was observed in male or female Fischer 344 rats, male or female CD-1 mice, or male Sprague-Dawley rats fed atrazine at a maximum tolerated level in their diet for 24 mo. Female Sprague-Dawley rats fed atrazine at levels of 400, 500, and 1000 ppm developed mammary tumors earlier than did the control group. The incidence of female Sprague-Dawley rats with mammary tumors after 24 mo of treatment was statistically increased at feeding levels of > or = 70 ppm in 1 study and at 400 ppm in a second study, whereas there were no significant differences between the treated and the control group in 3 other studies. No increase in tumors of any type was observed in ovariectomized female Sprague-Dawley rats after 24 mo of atrazine treatment at the highest level tested, 400 ppm. Therefore, the mammary tumor response in female Sprague-Dawley rats following the administration of high levels of atrazine appears to be due to an acceleration of the normal reproductive aging process resulting in increased exposure to endogenous estrogen and prolactin. The Sprague-Dawley rat differs from the Fischer 344 rat, the CD-1 mouse, and humans in the endocrine control mechanisms affecting reproductive senescence and the development of the mammary tumors during aging. These data indicate that the carcinogenic effect of high doses of atrazine observed in the female Sprague-Dawley is a strain-, sex-, and tissue-specific response that does not have biological relevance to humans.

Adequacy of required regulatory hazard testing for the detection of potential hormonal activity of crop protection chemicals

The capacity of some synthetic chemicals, the so-called "endocrine-disrupting chemical," to alter hormonal activity, as well as the adequacy of the testing of chemicals to evaluate this capacity, has been called into question. Among the chemicals indicted have been certain crop protection agents or pesticides. Crop protection chemicals rank among the most closely regulated and thoroughly tested chemicals in use in both the human health and environmental hazard areas. However, it has been proposed that in vitro and in vivo screening tests be used to identify potential endocrine-active chemicals and to supplement or replace required regulatory bioassays. In vitro tests, such as receptor binding, examine a single chemical event, do not measure toxicity, post-receptor-mediated biological response, or the absorption, distribution, metabolism, and elimination of a chemical. Therefore, data derived solely from such a limited testing technique should not be used as a basis for selection of chemicals for making regulatory decisions. In vivo screening tests, such as the uterotrophic assay, which promise to provide a rapid answer to a targeted question, do not capture the complexity of the biological response. As in the case with in vitro tests, results from a single in vivo test, such as a change in uterine weight, should not be used as a basis for regulatory decision making. Further, it has been suggested that such a screening battery should be put into place for ecotoxicity testing. Yet it is well recognized that endocrine-active chemicals that affect fish and wildlife in their natural habitat have been shown to cause similar adverse effects in laboratory test animals. Therefore, these screening tests do not add value to the current regulatory test battery. Evidence is presented that demonstrates that the regulatory safety assessment paradigm has a low likelihood of missing potential endocrine-active chemicals and has served society well.

FIFRA Subdivision F testing Guidelines: are these tests adequate to detect potential hormonal activity for crop protection chemicals? Federal Insecticide, Fungicide, and Rodenticide Act

Recently, a major topic of discussion has been the impact of synthetic chemicals that possess the capacity to alter hormonal activity, the so-called "endocrine modulators," with potentially the capacity to alter the reproductive capability of humans. Particularly, various synthetic pesticides and industrial chemicals that persist in the environment and/or bioaccumulate have been implicated. Further, it has been alleged that the standard tests for pesticide registration as required by the U.S. Environmental Protection Agency (EPA) and other regulatory agencies may be inadequate to detect endocrine modulating effects. To address these shortcomings, it has been proposed that very specific tests for estrogen receptor binding, or in vitro cell response to chemicals, be used to identify potential endocrine modulators. However, such approaches have certain flaws that limit their application as screens. First, very specific tests, like receptor binding, evaluate only a single chemical event per test. Such tests do not measure toxicity or biological response. Isolated systems are very important for studying mechanisms of action or structure activity relationships, but can only provide a preliminary screen for a single mechanism of toxicity. Isolated systems can not be used to regulate a chemical without additional information. Second, they fail to test many other parts of the neuroendocrine control of the reproductive system. Testing for adverse effects in highly specific in vitro systems failed to replace whole-animal models in carcinogenesis and will also fail in reproductive toxicology because this system is too complicated for such as in vitro approach to be accurately predictive. Advanced tests, such as the EPA multigeneration study, are more effective, and reliable means for evaluation than any specific and narrowly focused screening tests. Experience has shown that a better approach to testing chemicals is to evaluate their effects on the whole animal. When one part of the system is adversely affected, various processes may be indirectly affected and can be detected in the animal model. For example, a modulation of testosterone synthesis could lead to (1) altered accessory sex organ morphology, size, and function; (2) decreased sperm counts; and (3) even decreased fertility. These and many other effects would be noted in toxicity studies that are already required for the registration of crop protection chemicals. The developmental and reproductive toxicity guidelines were recently reviewed in a hearing that included the representatives from the EPA, the public, and the Scientific Advisory Panel. The EPA kept the basic study design the same, but added a few new endpoints to further assess chemical-induced effects on reproductive development and function. The review presented herein concentrates on the required Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) testing for pesticides, and demonstrates how the massive arrays of sensitive endocrine endpoints that are delineated in FIFRA Subdivision F have been successfully used to detect both weak and potent hormonally modulating chemicals. For example, (1) diethyl-stilbestrol (DES), which is a potent synthetic therapeutic estrogen, (2) DDT, which is weakly estrogenic but persistent and bioaccumulating, and (3) dioxins, which have antiestrogenic properties, were all found as being hormonally active in tests similar or identical to FIFRA tests. All food-use pesticides have been evaluated using a comprehensive multigeneration reproduction test. Hence, the FIFRA testing procedures have been demonstrated to identify endocrine modulators of sufficient potency to represent a concern to human health.

Endocrine modulation of reproduction

The ability of foreign compounds to affect the functioning of various endocrine systems is currently thought responsible for a wide variety of effects. The presentations in this Symposium reviewed the evidence for and against the involvement of endocrine systems in several different aspects of reproduction. The mechanism behind the ability of a triazine herbicide to cause enhanced appearance of mammary tumors in one strain of female rats is reviewed by Stevens. The data suggest that enhanced aging, not direct mammary modulation, is responsible. Dietary phytoestrogens, the mediators of their actions, their effects in various biological systems, and the relationships between phytoestrogen producers and consumers are all provocatively and succinctly reviewed by Hughes. Kelce presents the strategy used to dissect the mode and mechanisms of action of a fungicide that opened a new awareness in reproductive toxicology: the possibility of xenobiotics being antiandrogens. Finally, to heighten our understanding of the interplay among hormonal systems in vivo, Hess reviews the data that show that androgens are not the only hormones important in the development of the male reproductive system: the pituitary is shown to play a critical role at specific stages of development. The breadth of these presentations, and the implications of their findings, should make us pause and realize how much there is still to discover about the interaction between the reproductive system and anthropogenic compounds.

Characterization of monoclonal antibodies recognizing amino- and carboxy-terminal epitopes of the herpes simplex virus UL42 protein

A panel of monoclonal antibodies (MAbs) directed against the herpes simplex virus type 1 (HSV-1) DNA polymerase (Pol) accessory protein, UL42, was developed and characterized. Thirteen different MAbs were isolated which exhibited varied affinities for the protein. All MAbs reacted with UL42 in ELISA, Western blot and immunoprecipitation analyses. Competitive ELISA was used to show that 6 different epitopes within UL42 were recognized by the MAbs. Immunoprecipitation of amino- and carboxy-terminal truncations of UL42 mapped the epitopes to regions containing amino acids 1-10, 10-108, 338-402, 402-460, and 460-477. All but one of these epitopes were outside the minimal active portion of the protein previously mapped to amino acids 20-315. None of these MAbs, alone or in combination, specifically neutralized the ability of UL42 to stimulate Pol activity in vitro. These results are consistent with structure-function studies that showed that N- and C-terminal regions of the UL42 protein, those recognized by the MAbs, are not involved in UL42 function in vitro.

The effect of internal autocleavage on kinetic properties of the human cytomegalovirus protease catalytic domain

The 28-kilodalton (kDa) catalytic domain of the human cytomegalovirus (HCMV) protease undergoes autoproteolytic cleavage at an internal site (I site), yielding amino-terminal 15-kDa (N15) and carboxyl-terminal 13-kDa (C13) fragments. I site autocleavage has been postulated to inactivate the protease and provide a mechanism for the negative regulation of enzyme activity during viral infection. We purified recombinant enzymes to demonstrate I site autocleavage in vitro and used site-directed mutagenesis of the I site to stabilize the protease. No difference in the kinetic properties of wild type and stabilized mutant proteases were observed in an in vitro peptide cleavage assay. The consequences of I site cleavage on enzyme activity were investigated two ways. First, autodigestion of the wild type enzyme converted the intact protease to N15 and C13 autocleavage products without a corresponding loss in enzyme activity. Second, genetic constructs encoding the N15 and C13 autocleavage products were generated and expressed separately in Escherichia coli, and each fragment was purified. An active enzyme was reconstituted by refolding a mixture of the purified fragments in vitro to form a noncovalent complex. The kinetic properties of this complex were very similar to the wild type and stabilized enzymes under optimal reaction conditions. We concluded from these studies that I site cleavage does not inactivate the HCMV protease, in the absence of other virally induced factors, and that limited potential exists for the regulation of catalytic activity by I site cleavage.

Pharmacokinetic factors influencing risk assessment: saturation of biochemical processes and cofactor depletion

Models generally consider risk to be a function of the hazard (toxicity) and exposure (dose). That function is best described by the dose response of the toxic effect. For any risk assessment system to be effective, it should consider that dose-response relationship. Saturation phenomena often produce nonlinear dose curves, and any risk assessment system should be able to address such effects. Physiologically based pharmacokinetics offer an approach to deal with these nonlinear responses. Some historic risk models and common saturable processes are discussed. The impact of maximum tolerated dose (MTD) on risk evaluation and the kinetics of some saturable processes are considered. Specific examples have been selected to demonstrate the importance of saturation of processes in assessing the hazard of chemicals.

In vitro proteolytic activity and active-site identification of the human cytomegalovirus protease

Human cytomegalovirus (HCMV) encodes a protease that cleaves itself and the HCMV assembly protein. Two proteolytic processing sites within the protease were identified at Ala 256-Ser 257 (release site) and Ala 643-Ser 644 (maturation site). Identification of rP5-P4' and mP4-P6' as the minimal peptide substrates spanning the release and maturation cleavage sites, respectively, demonstrated a requirement for residues flanking the conserved core in substrate recognition and hydrolysis, which are unique to HCMV. Kinetic parameters determined for release-site-derived and maturation-site-derived peptides revealed a 10-fold increase in kcat/Km for a maturational peptide (mP4-P8') over release-site peptide (rP5-P5'). Experimental results with a panel of class-specific protease inhibitors were consistent with the protease being a member of the serine or cysteine family of proteases. Further investigation revealed that the HCMV protease activity decreased with incorporation of [14C]iodoacetic acid, but when approximately 4.5 mol 14C were incorporated/mol enzyme, the enzyme retained approximately 20% of its original activity, indicating that hydrolysis does not require a cysteine nucleophile. Analysis of diisopropyl-fluorophosphate-inactivated protease by mass spectrometry indicated a stoichiometry of 1 diisopropyl phosphate/protease molecule, suggesting that hydrolysis requires a single serine nucleophile. The residue modified by diisopropyl fluorophosphate was identified as Ser132 by modification with 3H-labeled diisopropyl fluorophosphate, peptide mapping and Edman degradation. This residue and the region in which it is found is highly conserved among the herpes virus proteases. These data demonstrates that HCMV protease is a serine protease and that Ser132 is the active-site nucleophile.

Factors affecting mammary tumor incidence in chlorotriazine-treated female rats: hormonal properties, dosage, and animal strain

Chlorotriazines are widely used in agriculture as broadleaf herbicides. The compounds specifically inhibit photosynthesis, and, as such, display little interaction with animal systems. However, a 24-month feeding study with atrazine (ATR) revealed a significant dose-related increase of mammary tumors in female Sprague-Dawley (SD) rats. Because numerous studies indicated that ATR had a low mutagenic and oncogenic potential, it was decided to test a hypothesis that the herbicide possessed endocrine activity. Among tests for estrogenic action, oral dosing of ATR up to 300 mg/kg did not stimulate uterine weight of ovariectomized rats. However, ATR administration did reduce estrogen-stimulated uterine weight gain. Further evidence of inhibition came from measures of [3H]-thymidine incorporation into uterine DNA of ATR-treated immature rats. Again, no intrinsic estrogenic activity was observed up to a 300-mg/kg dose. In vitro, ATR competed poorly against estradiol binding to cytosolic receptors, with an approximate IC50 of 10(-5) M. Atrazine administration to SD and Fischer-344 (F-344) rats for 12 months, up to 400 ppm in food, was correlated with significant alterations of estrous cycling activity; but there was a divergent strain response. SD rats showed an increased number of days in vaginal estrus, increased plasma estradiol, and decreased plasma progesterone by 9 to 12 months of treatment. F-344 rats did not demonstrate treatment-related affects. A study of ultrastructure in the hypothalamic arcuate nucleus of female SD rats that were fed diaminochlorotriazine (DACT), an ATR metabolite, suggested that age-associated glial pathology was enhanced by treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible antiestrogenic properties of chloro-s-triazines in rat uterus

Several published reports have indicated that certain chloro-s-triazine herbicides may alter endocrine function in rats, possibly by androgen receptor binding. In direct tests of estrogenic bioactivity, oral doses of up to 300 mg/kg/d of atrazine, simazine, or the common metabolite diaminochlorotriazine (DACT) did not significantly increase uterine weight of ovariectomized Sprague-Dawley female rats. The highest dose, which was approximately 10% of the LD50 for these compounds, did cause body weight loss. When administered concomitantly with sc injections of estradiol (2 micrograms/kg), 300 mg/kg of orally administered chlorotriazines significantly reduced uterine weight in comparison to animals given estrogen alone. Neither atrazine, simazine, nor DACT, at oral doses up to 300 mg/kg/d, stimulated incorporation of [3H]thymidine into uterine DNA of immature Sprague-Dawley female rats. However, oral treatment at doses of 50 mg/kg and higher significantly reduced thymidine incorporation into uterine DNA extracted from immature rats given a single injection of 0.15 microgram estradiol. Oral doses of 300 mg/kg of atrazine, simazine, or DACT significantly reduced expression of progesterone receptor binding in cytosol fractions prepared from uteri of ovariectomized rats injected sc with 1 microgram estradiol; 50 mg/kg triazine was not effective in this case. Uterine progesterone receptor levels were not stimulated in rats given oral doses up to 300 mg/kg of these triazines without estradiol injections. These results suggest that atrazine, simazine, and DACT possess no intrinsic estrogenic activity but that they are capable of weak inhibition of estrogen-stimulated responses in the rat uterus. This inhibition may play a role in the previously observed disruptive actions of chlorotriazines on reproductive endocrine function of female rats.

Short-term effects of chlorotriazines on estrus in female Sprague-Dawley and Fischer 344 rats

Atrazine or simazine (s-chlorotriazines) was administered by gavage daily for 2 wk to female Sprague-Dawley and Fischer 344 rats at oral doses of 100 or 300 mg/kg to evaluate effects on body, ovary, uterus, and adrenal weights, estrous cycle stages, vaginal cytology, and plasma hormone (estradiol, progesterone, prolactin, and corticosterone) levels. Significant reductions in body weights of both Sprague-Dawley and Fischer 344 female rats at both dose levels were accompanied by a significant reduction in ovarian and uterine weights, and a decrease in circulating estradiol levels. The magnitudes of the effects were less in Fischer 344 rats than in Sprague-Dawley rats, and the effects of simazine were less pronounced than those of atrazine at the same dose. A maximum tolerated dose (MTD: > or = 10% body weight reduction) was estimated to be 100 mg/kg for atrazine and 300 mg/kg for simazine for both stains. The Sprague-Dawley female rats exhibited a treatment-related lengthening of the estrous cycle and an increased number of days characterized by cornified epithelial cells. This resulted in a greater percent of the cycle days spent in estrus and reduction in the percent of the cycle days spent in diestrus. Atrazine-dosed Fischer 344 females also exhibited a significant trend toward cycle lengthening, but this was due to reduction in the percent of cycle spent in estrus and a concomitant increase in diestrual days. These findings suggest that treatment with doses of triazine at or above the MTD may result in prolonged exposure to endogenous estrogen in the Sprague-Dawley but not the Fischer 344 rat. These changes may account for the observed earlier onset and/or increased incidence of mammary tumors in chlorotriazine-treated female Sprague-Dawley rats. This strain of rat is already known to be prone to a substantial development of mammary tumors with advancing age, while the Fischer 344 strain is not as likely to exhibit this response.

Chloro-s-triazine antagonism of estrogen action: limited interaction with estrogen receptor binding

In an accompanying article (see pp. 183-196), it was reported that administration of very high doses of the chlorotriazine herbicides atrazine, simazine, and diaminochlorotriazine (DACT), a common metabolite, expressed antiestrogenic activity in uteri of female Sprague-Dawley rats without expressing intrinsic estrogenic activity. In the present article, studies of chlorotriazine interaction with rat uterine estrogen receptors (ER) are reported. Under equilibrium conditions, none of the triazine compounds showed an ability to compete against binding of radiolabeled estradiol to ER. A weak competition was evident only if cytosols were preincubated with triazines at 25 degrees C prior to introduction of tracer. Competition was very weak, with kl estimates of 10-100 microM. A limited Scatchard analysis suggested a competitive type of inhibition. Sucrose gradient analysis of cytosol incubations showed that triazine interaction with the 4S isoform of ER may be greater than with the 8S form. When administered to ovariectomized rats for 2 d at 300 mg/kg/d, atrazine, simazine, or DACT all reduced uterine ER binding capacity by approximately 30%. Results from the receptor binding studies indicated that triazine competition against ER binding occurred to a much lesser degree than inhibition of estrogen-mediated responses reported in accompanying articles. This suggests that the complete responses to triazines may include inhibition of events other than or in addition to ER binding of estrogen.

Chronic effects of atrazine on estrus and mammary tumor formation in female Sprague-Dawley and Fischer 344 rats

The chronic effects of dietary administration of atrazine at levels as high as 400 ppm on selected endocrine and tumor profiles were evaluated in Fischer 344 and Sprague-Dawley female rats. The study showed that lifetime dietary administration of atrazine at a maximum tolerated dose (MTD) to Sprague-Dawley female rats caused (1) lengthening of the estrous cycle, (2) increased number of days in estrus or under the influence of exposure to estrogen, (3) earlier onset of galactocele formation, and (4) earlier onset of mammary and pituitary tumor formation but not an increased incidence of mammary and pituitary tumors when compared to concurrent control rats. Fischer 344 female rats fed atrazine at an MTD exhibited slightly lengthened estrous cycles, but no effects were observed on estradiol or progesterone levels, or on the onset or incidence of mammary tumors. These results support a hypothesis that high-dose atrazine administration in Sprague-Dawley females is related to an acceleration of age-related endocrine changes leading to an earlier onset and/or increased incidence of mammary tumors. This endocrine-mediated response, which appears to be unique to the Sprague-Dawley female rat, occurs only at or above a threshold dose (the MTD) that interferes with normal estrous cycling, promoting prolonged exposure to endogenous estrogen.

Hypothesis for mammary tumorigenesis in Sprague-Dawley rats exposed to certain triazine herbicides

The symmetrical triazine herbicides have been used for the preemergence control of broadleaf weeds for nearly three decades. Recently, certain members of this class, primarily the chlorotriazines (substituted in the 2 position), have been shown to evoke an increased incidence of mammary tumors in female Sprague-Dawley rats. This response was noted when these chemicals were administered in the diet for 2 yr, and most often at dietary feeding levels at or above the maximum tolerated dose (MTD). At levels exceeding the MTD the health of these animals was compromised, as manifested by toxicity-related reduced survival that was not associated with the occurrence of mammary tumors. Mammary tumors in rats frequently occur as a result of the influence of endogenous estradiol and prolactin. Those hormones, as well as progesterone, growth-stimulating, luteinizing, and follicle-stimulating hormones, were measured after 24 mo of dietary administration of the chlorotriazine, simazine. The plasma hormone pattern seen in aged female Sprague-Dawley rats administered 1000 ppm simazine in the diet for 24 mo resembled that noted for aged female controls, except that the difference was more pronounced in the simazine-treated group. These results suggest that prolonged exposure of Sprague-Dawley females to excessive levels of triazines affects the neuroendocrine system, which in turn alters the pathology of the mammary gland. These changes are comparable to those that occur naturally as the rat ages. Changes in neuroendocrine control could result in the expression of an earlier onset and/or an increased incidence of mammary tumors, which already occur at a high spontaneous rate in aging Sprague-Dawley female rats.

Identification of the serine residue at the active site of the herpes simplex virus type 1 protease

Herpes simplex virus type 1 (HSV-1) encodes a protease that is essential for proteolytic processing of itself and of the nucleocapsid-associated protein, ICP35 (infected cell protein 35) (Liu, F., and Roizman, B. (1991) J. Virol. 65, 5149-5156). Inhibitor studies indicated that the HSV-1 protease is sensitive to the serine protease inactivator diisopropyl fluorophosphate (DFP). Inactivation is irreversible and dependent on time and concentration of DFP. Loss of activity correlates linearly with the incorporation of [3H]DFP. Analysis of completely inactivated protease by mass spectrometry indicated a stoichiometry of 1 DFP/protease. In order to identify the specific residue modified by DFP, the protease was labeled with [3H]DFP and subsequently digested with trypsin or chymotrypsin. The peptides resulting from each digestion were separated by reverse phase HPLC, and the radioactivity was recovered in a single peak. Mass spectrometric studies and sequencing analysis by Edman degradation identified Ser-129 as the residue modified by DFP. This residue and the region in which it is found is highly conserved among the herpes viral proteases. These data demonstrate that HSV-1 protease is a serine protease and that Ser-129 is the active site nucleophile.

Autoproteolysis of herpes simplex virus type 1 protease releases an active catalytic domain found in intermediate capsid particles

The UL26 gene of herpes simplex virus type 1 (HSV-1) encodes a 635-amino-acid protease that cleaves itself and the HSV-1 assembly protein ICP35cd (F. Liu and B. Roizman, J. Virol. 65:5149-5156, 1991). We previously examined the HSV protease by using an Escherichia coli expression system (I. C. Deckman, M. Hagen, and P. J. McCann III, J. Virol. 66:7362-7367, 1992) and identified two autoproteolytic cleavage sites between residues 247 and 248 and residues 610 and 611 of UL26 (C. L. DiIanni, D. A. Drier, I. C. Deckman, P. J. McCann III, F. Liu, B. Roizman, R. J. Colonno, and M. G. Cordingley, J. Biol. Chem. 268:2048-2051, 1993). In this study, a series of C-terminal truncations of the UL26 open reading frame was tested for cleavage activity in E. coli. Our results delimit the catalytic domain of the protease to the N-terminal 247 amino acids of UL26 corresponding to No, the amino-terminal product of protease autoprocessing. Autoprocessing of the full-length protease was found to be unnecessary for catalysis, since elimination of either or both cleavage sites by site-directed mutagenesis fails to prevent cleavage of ICP35cd or an unaltered protease autoprocessing site. Catalytic activity of the 247-amino-acid protease domain was confirmed in vitro by using a glutathione-S-transferase fusion protein. The fusion protease was induced to high levels of expression, affinity purified, and used to cleave purified ICP35cd in vitro, indicating that no other proteins are required. By using a set of domain-specific antisera, all of the HSV-1 protease cleavage products predicted from studies in E. coli were identified in HSV-1-infected cells. At least two protease autoprocessing products, in addition to fully processed ICP35cd (ICP35ef), were associated with intermediate B capsids in the nucleus of infected cells, suggesting a key role for proteolytic maturation of the protease and ICP35cd in HSV-1 capsid assembly.

Deletions of the carboxy terminus of herpes simplex virus type 1 UL42 define a conserved amino-terminal functional domain

The herpes simplex virus type 1 UL42 protein was synthesized in reticulocyte lysates and assayed for activity in vitro. Three functional assays were used to examine the properties of in vitro-synthesized UL42: (i) coimmunoprecipitation to detect stable complex formation with purified herpes simplex virus type 1 DNA polymerase (Pol), (ii) a simple gel-based assay for DNA binding, and (iii) a sensitive assay for the stimulation of Pol activity. UL42 synthesized in reticulocyte lysates formed a stable coimmunoprecipitable complex with Pol, bound to double-stranded DNA, and stimulated the activity of Pol in vitro. Carboxy-terminal truncations of the UL42 protein were synthesized from restriction enzyme-digested UL42 gene templates and gene templates made by polymerase chain reaction and assayed for in vitro activity. Truncations of the 488-amino-acid (aa) UL42 protein to aa 315 did not abolish its ability to bind to Pol and DNA or to stimulate Pol activity. Proteins terminating at aas 314 and 313 showed reduced levels of binding to Pol, but these and shorter proteins were unable to bind to DNA or to stimulate Pol activity. These results suggest that all three of the biochemical functions of UL42 colocalize entirely within the N-terminal 315 aas of the UL42 protein. Amino acid sequence alignment of alpha herpesvirus UL42 homologs revealed that the N-terminal functional domain corresponds to the most highly conserved region of the protein, while the dispensable C terminus is not conserved. Conservative aa changes at the C terminus of the 315-aa truncated protein were used to show that conserved residues were important for activity. These results suggest that 173 aa of UL42 can be deleted without a loss of activity and that DNA-binding and Pol-binding activities are correlated with the ability of UL42 to stimulate Pol activity.

Mutations in the C terminus of herpes simplex virus type 1 DNA polymerase can affect binding and stimulation by its accessory protein UL42 without affecting basal polymerase activity

We have analyzed the effects of mutations in the herpes simplex virus type 1 DNA polymerase (Pol) C-terminal UL42 binding domain on the activity of Pol and its ability to form complexes with and be stimulated by UL42 in vitro. Wild-type Pol expressed in Saccharomyces cerevisiae was both bound and stimulated by UL42 in vitro. C-terminal truncations of 19 and 40 amino acids (aa) did not affect the ability of Pol to be stimulated by UL42 in vitro. This stimulation as well as basal Pol activity in the presence of UL42 was inhibited by polyclonal anti-UL42 antiserum, thus indicating a physical interaction between Pol and UL42. Removal of the C-terminal 59 aa of Pol and internal deletions of 72 aa within the Pol C terminus eliminated stimulation by UL42. None of the truncations or deletions within Pol affected basal polymerase activity. In contrast with their ability to be stimulated by UL42, only wild-type Pol and Pol lacking the C-terminal 19 aa bound UL42 in a coimmunoprecipitation assay. These results demonstrate that a functional UL42 binding domain of Pol is separable from sequences necessary for basal polymerase activity and that the C-terminal 40 aa of Pol appear to contain a region which modulates the stability of the Pol-UL42 interaction.

Comparison of shear bond strengths of some third-generation dentin bonding agents

Recently a series of new dentin bonding systems has been introduced to the dental profession. These agents are the third generation of systems that have been developed over the past two decades. Some dentin bonding agents are easily applied, clinically, while others are complicated, multi-step procedures. A comparison of the shear bond strengths of five of these systems was made at 15 minutes and after 24 hours stored in water at 37 degrees C.

Structure-function studies of the herpes simplex virus type 1 DNA polymerase

The analysis of the deduced amino acid sequence of the herpes simplex virus type 1 (HSV-1) DNA polymerase reported here suggests that the polymerase structure consists of domains carrying separate biological functions. The HSV-1 enzyme is known to possess 5'-3'-exonuclease (RNase H), 3'-5'-exonuclease, and DNA polymerase catalytic activities. Sequence analysis suggests an arrangement of these activities into distinct domains resembling the organization of Escherichia coli polymerase I. In order to more precisely define the structure and C-terminal limits of a putative catalytic domain responsible for the DNA polymerization activity of the HSV-1 enzyme, we have undertaken in vitro mutagenesis and computer modeling studies of the HSV-1 DNA polymerase gene. Sequence analysis predicts that the major DNA polymerization domain of the HSV-1 enzyme will be contained between residues 690 and 1100, and we present a three-dimensional model of this region, on the basis of the X-ray crystallographic structure of the E. coli polymerase I. Consistent with these structural and modeling studies, deletion analysis by in vitro mutagenesis of the HSV-1 DNA polymerase gene expressed in Saccharomyces cerevisiae has confirmed that certain amino acids from the C terminus (residues 1073 to 1144 and 1177 to 1235) can be deleted without destroying HSV-1 DNA polymerase catalytic activity and that the extreme N-terminal 227 residues are also not required for this activity.

Neutralization of purified herpes simplex virus DNA polymerase by two antipeptide sera

Synthetic peptides corresponding to amino acid sequences present in the herpes simplex virus type-1 (HSV-1) DNA polymerase (pol) were used to raise polyclonal rabbit antisera. The three peptides described in detail in this report were among seven sequences chosen for initial studies designed to generate reagents capable of recognizing discrete regions of the HSV-1 pol protein from the amino to carboxy termini. Two of the peptides, designated P6 and P7, representing amino acid residues 1100-1108 and 1216-1224 of the deduced HSV-1 (strain KOS) DNA pol sequence (1235 residues) produced antisera that could not only recognize the native HSV-1 pol enzyme but also could specifically neutralize purified HSV-1 pol activity in a dose-dependent manner. An additional peptide, designated P3, representing residues 548-557, produced an antiserum that was unable to recognize the native protein but could react with HSV-1 pol in a denatured form by immunoblot assay.

In vitro mutagenesis of the herpes simplex virus type 1 DNA polymerase gene results in altered drug sensitivity of the enzyme

A mutation (asparagine 815 to serine 815) was introduced into the herpes simplex virus type 1 (HSV-1) DNA polymerase (pol). The HSV-1 pol enzyme in lysates of Saccharomyces cerevisiae cells expressing the mutant protein showed increased resistance to acyclovir triphosphate and increased sensitivity to phosphonoacetate but was not substantially altered with respect to sensitivity to phosphonoformate or aphidicolin. These results directly demonstrate that both resistance to acyclovir triphosphate and sensitivity to phosphonoacetate can be conferred by this mutation in the absence of other viral factors and that the yeast expression system can be used for structure-function studies on HSV-1 pol.

Expression of herpes simplex virus type 1 DNA polymerase in Saccharomyces cerevisiae and detection of virus-specific enzyme activity in cell-free lysates

The herpes simplex virus type 1 (HSV-1) (strain 17) DNA polymerase gene has been cloned into an Escherichia coli-yeast shuttle vector fused to the galactokinase gene (GAL-1) promoter. Genes controlled by the GAL-1 promoter are induced by galactose, uninduced by raffinose, and repressed by glucose. Cell extracts from a strain of Saccharomyces cerevisiae harboring this vector (Y-MH202, expresser cells) grown in the presence of galactose and assayed in high salt (100 mM ammonium sulfate) contained a novel DNA polymerase activity. No significant high-salt DNA polymerase activity was detected in extracts from expresser cells grown in the presence of raffinose or in extracts from control cells containing the E. coli-yeast shuttle vector without the HSV-1 DNA polymerase gene grown in the presence of raffinose of galactose. Immunoblot analysis of the cell extracts by using a polyclonal rabbit antiserum prepared against a highly purified HSV-1 DNA polymerase preparation revealed the specific induction of the HSV-1 approximately 140-kilodalton DNA polymerase polypeptide in expresser cells grown in galactose. Extracts from the same cells grown in raffinose or control cells grown in either raffinose or galactose did not contain this immunoreactive polypeptide. The high-salt DNA polymerase activity in the extracts from expresser cells grown in galactose was inhibited greater than 90% by either acyclovir triphosphate or aphidicolin, as expected for HSV-1 DNA polymerase. In addition, the high-salt polymerase enzyme activity could be depleted from extracts by immunoprecipitation by using purified immunoglobulin G from this same polyclonal rabbit antiserum. These results demonstrate the successful expression of functional HSV-1 DNA polymerase enzyme in S. cerevisiae.

The importance of metabolite identification in quantitative risk estimation

In the search to define the mechanisms by which xenobiotics produce their toxic effects in biological systems, the importance of metabolism data is clear. Although the detection of electrophilic metabolites and reactive intermediates may challenge our analytical technology, the toxic responses manifested by these agents are often obvious. The identification of toxicologically significant minor metabolites may exceed the state of the art in analytical methodology. New advances in technology may provide the needed answers. As we begin to face the significance of activation reactions, particularly in the area of carcinogenesis, it becomes apparent that metabolism to electrophiles that react covalently with DNA, is not the only mechanism by which the tumorigenic response is produced. The production of tumors by nongenotoxic (epigenetic) means is also important. Exposure to high and sustained levels of exposure to a xenobiotic that leads to a perturbation in metabolic, endocrine or physiologic pathways or tissue injury may also produce tumors. Only through investigations which include definitive metabolite identification and quantitation can the mechanism by which these agents exert their toxicity be identified. The ramification of dose response relationships for genotoxic and nongenotoxic carcinogens will be presented to demonstrate the impact of metabolite identification in quantitative risk estimation.

Inhibition of p-aminophenol metabolism: a possible mechanism of enhancement of aniline hydroxylation

The p-hydroxylation of aniline has been traditionally determined by measurement of p-aminophenol (PAP) formation. Comparison of isotopic and colorimetric procedures indicate that the actual amount of aniline metabolized exceeds the among of PAP recovered. Data suggest that enhancement of p-hydroxylation of aniline by acetone, malaoxon and paraoxon may result from inhibition of further metabolism of PAP by the microsomal cytochrome b5-dependent desaturase system. Involvement of the desaturase system is supported by observations that: (a) metabolism of PAP was reduced by starvation and stimulated when starvation was followed by feeding a high carbohydrate diet; (b) enrichment of hepatic microsomes with detergent purified cytochrome b5 decreased the amount of aniline apparently metabolized, as measured by the amount of PAP recovered, and (c) a high correlation occurred between effects of acetone, malaoxon and paraoxon on reoxidation of cytochrome b5 and capacity of these three compounds to enchance aniline metabolism.

Interaction of pargyline with rat hepatic microsomes

When administered by intraperitoneal injection daily for 3, 7 or 14 days, pargyline (75 mg/kg) significantly reduced rat hepatic microsomal ethylmorphine N-demethylase activity and cytochrome P-450 content. Injection of a lower dose of pargyline (15 mg/kg) failed to alter significantly either ethylmorphine N-demethylase activity or cytochrome P-450 content. Studies performed in vitro reveal that pargyline is metabolized to at least three compounds by rat hepatic microsomes. Thin-layer chromatography and other analyses suggest that one metabolite is an N-demethylated form, norpargyline.

Disposition of 14C and/or 74As-cacodylic acid in rats after intravenous, intratracheal, or peroral administration

The distribution, excretion, and possible metabolism of (14)C- and/or (74)As-cacodylic acid, an organoarsenical herbicide, was studied in rats following a single intravenous injection, intratracheal instillation or oral gavage. Male Sherman rats were dosed at levels ranging from 200 mg/kg to 120 mug/kg. The extent and rate of lung absorption was greater than gastrointestinal absorption. Concentrations in the liver and whole blood were higher after peroral dosing than intravenous administration. Levels observed in plasma and other tissues were similar after all three routes following the absorptive phase. The percent dose found in the whole blood, red blood cells, and plasma was similar for all doses given by these routes. Less than 0.1(1/2) of the administered dose was recovered as (14)CO(2) by any route at 24 hr after administration. Twenty-four hours after intravenous, intratracheal, and peroral administration, 71, 60, and 25%, respectively, was excreted in the urine. After intravenous administration of 200 mg/kg, sufficient (14)C-cacodylic acid was recovered in bile to account for the small amount excreted in the feces. Cacodylic acid is probably not metabolized to inorganic arsenic since the disposition of (14)C and (74)As-cacodylic acid were identical.Kinetic analyses of the plasma curve for (14)C-cacodylic acid (high dose) yielded three half-times; 0.014, 0.214 and 3.42 hr with an apparent volume of distribution of 15.3 ml. Highest initial concentrations were found in the whole blood, muscle, kidney, liver and lung. Levels in all tissues decreased rapidly, but remained high in whole blood. The red blood cells were found to be the major site of body burden of cacodylic acid.