Free radical mechanisms in chemical pathogenesis. Summary of the symposium presented at the 1988 annual meeting of the Society of Toxicology

Early Chemistry of Nicotine Degradation in Heat-Not-Burn Smoking Devices and Conventional Cigarettes: Implications for Users and Second- and Third-Hand Smokers

Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine's degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation. The former contributes to the control of the degradation mechanism below 800 K due to a non-Arrhenius kinetics, which implies an enhancement of reactivity as temperature decreases. The latter becomes important at higher temperatures due to its larger activation energy. This change in the degradation mechanism is expected to affect the composition of vapors inhaled by smokers and room occupants. Conventional cigarettes, which operate at temperatures higher than 1000 K, are more prone to yield harmful pyridinyl radicals via nicotine dissociation, while nicotine in electronic cigarettes and vaporizers, with operating temperatures below 600 K, will be more likely degraded by hydroxyl radicals, resulting in a vapor with a different composition. Although low-temperature nicotine delivery devices have been claimed to be less harmful due to their nonburning operating conditions, the non-Arrhenius kinetics that we observed for the degradation mechanism below 873 K suggests that nicotine degradation may be more rapidly initiated as temperature is reduced, indicating that these devices may be more harmful than it is commonly assumed.

Kinetic modeling of nicotine in mainstream cigarette smoking

BACKGROUND

The attempt to understand the kinetic behavior of nicotine in tobacco will provide a basis for unraveling its energetics in tobacco burning and the formation of free radicals considered harmful to the cigarette smoking community. To the best of our knowledge, the high temperature destruction kinetic characteristics of nicotine have not been investigated before; hence this study is necessary especially at a time addiction science and tobacco research in general is gaining intense attention.

METHODS

The pyrolysis of tobacco under conditions simulating cigarette smoking in the temperature region 200-700 °C has been investigated for the evolution of nicotine and pyridine from two commercial cigarettes coded ES1 and SM1 using gas chromatography hyphenated to a mass selective detector (MSD). Moreover, a kinetic model on the thermal destruction of nicotine within a temperature window of 673 and 973 K is proposed using pseudo-first order reaction kinetics. A reaction time of 2.0 s was employed in line with the average puff time in cigarette smoking. Nonetheless, various reaction times were considered for the formation kinetics of nicotine.

RESULTS

GC-MS results showed the amount of nicotine evolved decreased with increase in the puff time. This observation was remarkably consistent with UV-Vis data reported in this investigation. Generally, the temperature dependent rate constants for the destruction of nicotine were found to be [Formula: see text] s^-1 and [Formula: see text] s^-1 for ES1 and SM1 cigarettes respectively. In addition, the amount of nicotine evolved by ES1 cigarette was ~10 times more than the amount of nicotine released by SM1 cigarette.

CONCLUSION

The suggested mechanistic model for the formation of pyridine from the thermal degradation of nicotine in tobacco has been found to be agreement with the kinetic model proposed in this investigation. Consequently, the concentration of radical intermediates of tobacco smoke such as pyridinyl radical can be determined indirectly from a set of integrated rate laws. This study has also shown that different cigarettes can yield varying amounts of nicotine and pyridine depending on the type of cigarette primarily because of potential different growing conditions and additives introduced during tobacco processing. The activation energy of nicotine articulated in this work is consistent with that reported in literature.Graphical abstractThe anatomy of tobacco cigarette and the major chemistry involved during combustion (pyrolysis, GC-MS analysis, and kinetic modeling).

Kinetic modeling of nicotine in mainstream cigarette smoking

BACKGROUND

The attempt to understand the kinetic behavior of nicotine in tobacco will provide a basis for unraveling its energetics in tobacco burning and the formation of free radicals considered harmful to the cigarette smoking community. To the best of our knowledge, the high temperature destruction kinetic characteristics of nicotine have not been investigated before; hence this study is necessary especially at a time addiction science and tobacco research in general is gaining intense attention.

METHODS

The pyrolysis of tobacco under conditions simulating cigarette smoking in the temperature region 200-700 °C has been investigated for the evolution of nicotine and pyridine from two commercial cigarettes coded ES1 and SM1 using gas chromatography hyphenated to a mass selective detector (MSD). Moreover, a kinetic model on the thermal destruction of nicotine within a temperature window of 673 and 973 K is proposed using pseudo-first order reaction kinetics. A reaction time of 2.0 s was employed in line with the average puff time in cigarette smoking. Nonetheless, various reaction times were considered for the formation kinetics of nicotine.

RESULTS

GC-MS results showed the amount of nicotine evolved decreased with increase in the puff time. This observation was remarkably consistent with UV-Vis data reported in this investigation. Generally, the temperature dependent rate constants for the destruction of nicotine were found to be [Formula: see text] s^-1 and [Formula: see text] s^-1 for ES1 and SM1 cigarettes respectively. In addition, the amount of nicotine evolved by ES1 cigarette was ~10 times more than the amount of nicotine released by SM1 cigarette.

CONCLUSION

The suggested mechanistic model for the formation of pyridine from the thermal degradation of nicotine in tobacco has been found to be agreement with the kinetic model proposed in this investigation. Consequently, the concentration of radical intermediates of tobacco smoke such as pyridinyl radical can be determined indirectly from a set of integrated rate laws. This study has also shown that different cigarettes can yield varying amounts of nicotine and pyridine depending on the type of cigarette primarily because of potential different growing conditions and additives introduced during tobacco processing. The activation energy of nicotine articulated in this work is consistent with that reported in literature.Graphical abstractThe anatomy of tobacco cigarette and the major chemistry involved during combustion (pyrolysis, GC-MS analysis, and kinetic modeling).

The role of biotransformation and bioactivation in toxicity

Biotransformation is essential to convert lipophilic chemicals to water-soluble and readily excretable metabolites. Formally, biotransformation reactions are classified into phase I and phase II reactions. Phase I reactions represent the introduction of functional groups, whereas phase II reactions are conjugations of such functional groups with endogenous, polar products. Biotransformation also plays an essential role in the toxicity of many chemicals due to the metabolic formation of toxic metabolites. These may be classified as stable but toxic products, reactive electrophiles, radicals, and reactive oxygen metabolites. The interaction of toxic products formed by biotransformation reactions with cellular macromolecules initiates the sequences resulting in cellular damage, cell death and toxicity.

The consequences of nitrofurantoin-induced oxidative stress in isolated rat hepatocytes: evaluation of pathobiochemical alterations

Oxidative stress was induced in isolated rat hepatocytes by incubation with nitrofurantoin in the absence and presence of the GSSG reductase inhibitor BCNU. In both cases nitrofurantoin markedly reduced glutathione but exerted cytotoxicity as measured by LDH release and loss of intracellular potassium only in BCNU pretreated cells. The onset of cytotoxicity was accompanied by an increase of lipid peroxidation. Oxidation of protein thiols, however, could not be detected in the early phase of cell damage. The cytoprotective activity of N-acetyl-cysteine > dithiothreitol = deferoxamine revealed the substantial importance of glutathione for cellular defence and the sensitivity of not yet identified thiol-dependent targets of oxidative stress.

Ultrasound-induced lung hemorrhage in the monkey

Studies with the mouse have shown that lung hemorrhage can result from exposure to ultrasound at a peak pressure of approximately 1 MPa at 4 MHz (Mechanical Index [MI] approximately 0.5). In order to determine whether a comparable outcome could occur in a larger animal with characteristics similar to humans, studies were performed with monkeys using a clinical scanner under maximum output conditions (imaging + pulsed and color Doppler; derated pr of 3.7 MPa [4.5 MPa, measured in water], 4 MHz; MI approximately 1.8) (N = 57). Monkeys ranged in age from 1 day of life to 16 years with exposures limited to the right lung lobes (5 min cranial, 5 min caudal; N = 41 exposed, N = 12 sham-exposed controls, N = 4 colony controls). Results showed that animals ranging in age from 3 months to 5 years (mean age of 2.5 years) had a greater propensity for the occurrence of multiple well-demarcated circular hemorrhagic foci (0.1-1.0 cm), which were not observed in either control group. These lesions were characterized by marked congestion of alveolar capillaries with accumulation of red blood cells within the alveolar spaces, and were unassociated with major vessels or respiratory bronchioles. Further studies will be required in order to determine the relevance of these findings to the human, although it was concluded that ultrasound-induced lung hemorrhage in the monkey is of a significantly lesser degree when compared to the mouse.

Morphological, biochemical, and histopathological indices and contaminant burdens of cotton rats (Sigmodon hispidus) at three hazardous waste sites near Houston, Texas, USA

Male cotton rats (Sigmodon hispidus) were studied at three industrial waste sites near Houston, Texas, to determine whether various morphological, biochemical, and histopathological indices provided evidence of contaminant exposure and toxic insult. Only modest changes were detected in cotton rats residing at waste sites compared with reference sites. No single parameter was consistently altered, except hepatic cytochrome P-450 concentration which was lower ( [Formula: see text] ) at two waste sites, and tended to be lower ( [Formula: see text] ) at a third waste site. Elevated petroleum hydrocarbon concentrations were detected in rats at one waste site, but contaminant burdens of rats from the other sites were unremarkable. Unlike rats captured in summer, those trapped in winter exhibited hepatocellular hypertrophy and up to a 65% increase in liver: body weight ratio, cytochrome P-450 concentration, and activities of aniline hydroxylase, aryl hydrocarbon hydroxylase, and glutathione S-transferase. Although genotoxicity has been previously documented in cotton rats residing at two of the waste sites, biomarkers in the present study provided little evidence of exposure and damage.

Alterations in the morphology and functional activity of bone marrow phagocytes following benzene treatment of mice

Benzene is a well-established hematotoxin that affects developing leukocytes and erythrocytes as well as bone marrow stromal cells. In the present studies we analyzed the effects of benzene on the morphology and functional activity of bone marrow phagocytes. Male Balb/c mice were treated with benzene (660 mg/kg) once per day for 3 days. Bone marrow cells were then isolated and fractionated by density gradient centrifugation. Using highly sensitive techniques in flow cytometry/cell sorting, we found that we could separate three distinct populations of bone marrow cells that differed with respect to size and density. Monoclonal antibody binding and cell sorting revealed a large, dense population that consisted predominantly of granulocytes, a smaller, less dense population of lymphocytes, and a population of intermediate size and density consisting of mononuclear phagocytes and precursor cells. Differential staining of sorted mononuclear phagocytes revealed that benzene treatment of mice caused a marked increase in the number of mature, morphologically activated macrophages in the bone marrow. Benzene treatment of mice also resulted in enhanced chemotaxis and production of hydrogen peroxide by bone marrow granulocytes and mononuclear phagocytes. In contrast, treatment of mice with the combination of hydroquinone and phenol (50 mg/kg each, 1 x/day, 3 days), two metabolites of benzene, resulted in a significant (p < or = 0.02) depression of granulocyte chemotaxis and had no effect on hydrogen peroxide production by bone marrow phagocytes compared to cells from control animals. Taken together these results demonstrate that benzene causes increased differentiation and/or activation of phagocytes in the bone marrow.

Oxygen radicals: common mediators of neurotoxicity

The inherent biochemical, anatomical and physiological characteristics of the brain make it especially vulnerable to insult. Specifically, some of these characteristics such as myelin and a high energy requirement provide for the introduction of free radical-induced insult. Recently, the biochemistry of free radicals has received considerable attention. It also has become increasingly apparent that many drug and chemical-induced toxicities may be evoked via free radicals and oxidative stress. Major points addressed in this work are the regulation of neural free radical generation by antioxidants and protective enzymes, xenobiotic-induced disruption of cerebral redox status, and specific examples of neurotoxic agent-induced alterations in free radical production as measured by the fluorescent probe dichlorofluorescein. This article considers the thesis that free radical mechanisms may contribute significantly to the properties of several diverse neurotoxic agents and proposes that excess production of free radicals may be common phenomena of neurotoxicity.

Hydrogen peroxide stimulates phospholipase A2-mediated arachidonic acid release in cultured intestinal epithelial cells (INT 407)

The mechanisms by which hydrogen peroxide and, for comparison, 4-beta-phorbol-12-myristate-13-acetate (PMA) stimulate release of radiolabeled arachidonic acid (14C-AA) in cultured intestinal epithelial cells (INT 407) were investigated. Both hydrogen peroxide and PMA caused a rapid (3 min) and dose-related intracellular release of free 14C-AA, followed by a dose- and time-dependent release of 14C-AA into the extracellular medium, but hydrogen peroxide was about 50,000 times less effective than PMA in releasing 14C-AA. No 14C-AA was released on stimulation with 4-alpha-phorbol-12,13-di-decanoate (PDD), a phorbol ester that does not activate protein kinase C. The 14C-AA release was reduced by the phospholipase A2 inhibitors nordihydroguaiaretic acid and 4-bromophenacyl bromide and by the calmodulin/protein kinase C inhibitor trifluoperazine and the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). However, H-7 was less effective than the other inhibitors in reducing the hydrogen peroxide-stimulated 14C-AA release. The hydrogen peroxide-stimulated, but not the PMA-stimulated, rapid (3 min) 14C-AA release was associated with an increased influx of extracellular calcium. Stimulation of the cells with PMA resulted in phosphorylation of a cellular protein of about 32 kDa, whereas no phosphorylation of this protein was detected after stimulation with hydrogen peroxide. Taken together, these findings indicate that (i) both PMA and hydrogen peroxide may stimulate phospholipase A2-mediated AA release from human intestinal epithelial cells; (ii) this stimulation is brought about via protein kinase C and calmodulin-mediated events; (iii) PMA-stimulated 14C-AA release is associated with phosphorylation of a 32-kDa protein, possibly lipocortin, whereas the hydrogen peroxide-stimulated release is not; and (iv) calmodulin is more important for the hydrogen peroxide-stimulated 14C-AA release than is protein kinase C. The possibility that hydrogen peroxide-evoked AA release may contribute to the mucosal abnormality in Crohn's disease is discussed.

Pulmonary lipid peroxidation in cigarette smokers and lung cancer patients

Lipid peroxidation (LPO) was studied in lung tissues of patients with lung cancer (LC, n = 37) or nonlung cancer (NLC, n = 13) and its relationships with the smoking habits and the degree of airway obstruction were investigated. Specimens of peripheral lung parenchyma, free of tumor tissue, were taken and the malondialdehyde (MDA) content was measured in the S-12 fractions. Airway obstruction was assessed by flow-volume curves, and data were expressed as percentage of the predicted values. Cigarettes smoked were expressed as pack-years. The patients with LC and NLC did not differ by MDA content, age, and number of pack-years. On the contrary, FEF75-85 and MEF75 were significantly lower in LC than in NLC patients (p less than 0.05). The MDA content was inversely correlated to number of days patients had refrained from smoking (r = -0.66, p less than 0.001). The MDA content was higher in recent smokers (ie, people smoking during the last 30 days before surgery) than in the other patients (0.136 +/- 0.007 vs 0.116 +/- 0.007 mumol/g of tissue, p less than 0.05) and, by considering only recent smokers, MDA content was higher in LC patients (0.144 +/- 0.008 mumol/g of tissue) than in NLC patients (0.113 +/- 0.014 mmol/g tissue, p = 0.059). When patients were divided into "high MDA" and "low MDA" groups, MEF75 was much lower in the high MDA group (35.1 +/- 3.4 percent) than in the low MDA group (55.1 +/- 8.1 percent) (p less than 0.01). These results suggest the following: (1) enhanced level of prooxidant state in the lungs is associated with recent cigarette smoking; (2) LC patients may be more prone than respective NLC patients to oxidative stress; (3) MDA level and degree of small airway obstruction were associated and differed between LC and NLC patients even though these groups did not differ in the percentage of recent smokers; and (4) a common free-radical mediated pathway may be active for both LC and small airway obstruction.

Increases in endogenous antioxidant enzymes during asbestos inhalation in rats

Although the pathogenesis of asbestos-induced pulmonary damage is still not completely understood, an important role has been attributed to active oxygen species. In the present paper we present results of a study investigating the effect of crocidolite asbestos inhalation on different lung antioxidant enzymes in rats. During the development of pulmonary fibrosis induced by crocidolite asbestos, lung superoxide dismutase, catalase and selenium-dependent glutathione peroxidase activities increased, indicating an adaptive response to increased pulmonary oxidant stress. However, this adaptive response obviously is not sufficient to protect the lung from asbestos-induced pulmonary damage. Considering the role of active oxygen species in both the fibrotic process and tumor promotion, it is hypothesized that antioxidants may also protect the lung from chronic asbestos-induced pulmonary damage such as bronchogenic carcinoma.

Characterization of the oxidant generation by inflammatory cells lavaged from rat lungs following acute exposure to ozone

Following exposure to 2 ppm ozone for 4 hr, two distinct effects on rat lung inflammatory cell oxidant generation were observed. TPA- and opsonized zymosan-stimulated superoxide production by the inflammatory cell population was found to be maximally inhibited 24 hr following ozone exposure. In contrast, luminol-amplified chemiluminescence increased 24 hr following ozone exposure, coinciding with an increase in the percentage of neutrophils and myeloperoxidase in the inflammatory cell population. Supporting the involvement of myeloperoxidase in the enhanced oxidant-generating status of these cells, the luminol-amplified chemiluminescence was found to be azide-, but not superoxide dismutase-inhibitable. Additionally, this cell population was found to generate taurine chloramines, a myeloperoxidase-dependent function which was absent prior to the ozone exposure and also demonstrated enhanced activation of benzo[a]pyrene-7,8-dihydrodiol to its light-emitting dioxetane intermediate. Addition of myeloperoxidase to control alveolar macrophages resulted in enhanced luminol-amplified chemiluminescence, taurine chloramine generation, and enhanced chemiluminescence from benzo[a]pyrene-7,8-dihydrodiol demonstrating that, in the presence of myeloperoxidase, alveolar macrophages are capable of supporting myeloperoxidase-dependent reactions. The possibility of such an interaction occurring in vivo is suggested by the detection of myeloperoxidase activity in the cell-free lavagates of ozone-exposed rats. These studies suggest that neutrophils recruited to ozone-exposed lungs alter the oxidant-generating capabilities in the lung which could further contribute to lung injury or to the metabolism of inhaled xenobiotics.