The distribution of benzo(a)pyrene and its metabolites in isolated perfused lung and liver

The formation of genotoxic metabolites of benzo[a]pyrene by the isolated perfused rat liver, as detected by the bioluminescence test

The kinetics of the formation of mutagenic metabolites of benzo[a]pyrene (BP) in an isolated perfused rat-liver system have been studied. No genotoxic activity was detected in the perfusate using either the Ames test or the new bioluminescence test for genotoxic agents (BLT). The bile excretion showed strong genotoxic activity especially in the presence of the deconjugation enzymes beta-glucuronidase and arylsulfatase. The BLT was 1000-fold more sensitive than the Ames test in detecting the genotoxic activity in the bile excretion.

Effects of cigarette smoke on rat lung and liver ornithine decarboxylase and aryl hydrocarbon hydroxylase activities and lung benzo(a)pyrene metabolism

The response of rat lung and liver ornithine decarboxylase (ODC) and aryl hydrocarbon hydroxylase (AHH) activities and lung benzo(a)pyrene (BP) metabolism was studied after exposing the rats to cigarette smoke. A close analysis of the time curves for ODC and AHH activities in rat lung and liver after a single exposure to cigarette smoke resulted in no clear correlation between the two parameters. Prolonged treatment (10 days) produced an increase in pulmonary ODC activity; hepatic ODC activity was unaffected. 10-day treatment was ineffective in raising AHH activity above values observed after a single treatment. BP metabolism, as determined in isolated perfused lungs by the appearance of organic- and water-soluble metabolites in the perfusion medium, the amount of covalently bound metabolites in lung tissue and the disappearance of unchanged 3-H BP from the perfusate, was markedly increased in response to cigarette smoke treatment. The data presented indicate that induction of AHH activity and increased metabolism of BP do not necessarily require a pre-existing increase in ODC activity.

The influence of flow on the metabolism of perfused benzo[a]pyrene by isolated rat lung

In order to investigate the influence of flow and, thus, substrate delivery, on the ability of lung to metabolize foreign compounds, the disappearance of circulating [3H]benzo[a]pyrene ([3H]B[a]P) and the appearance of B[a]P metabolites was monitored in isolated rat lungs from control and 3-methylcholanthrene (3-MC) pretreated rats perfused at low (10 ml/min) and high (45 ml/min) flows. Increasing the flow or 3-MC pretreatment hastened the disappearance of B[a]P from the perfusion medium reservoir and increased the rate of appearance of total metabolites. However, these manipulations affected the appearance of individual metabolites in the medium in different ways. For example, in lungs from control rats the rate of appearance of 7,8-dihydrodiol (7,8-dihydroxy-7,8-dihydro-B[a]P) (7,8-DHD) in the perfusion medium was markedly increased by increasing flow while that of B[a]P-1,6-quinone was minimally affected. In addition, increasing flow increased the concentration of some B[a]P metabolites, such as 4,5-dihydrodiol (4,5-dihydroxy-4,5-dihydro-B[a]P) (4,5-DHD) in the lung tissue of control rats at the end of the perfusion period, but did not effect much change in the concentration of these metabolites in lungs from 3-MC-pretreated rats. The results show that flow, as well as 3-MC pretreatment, may alter the rate at which metabolism of foreign compounds occurs and the temporal profile of metabolites produced by the intact lung.

Kinetics of uptake and biliary excretion of benzo(alpha)pyrene and mutagenic metabolites in isolated perfused rat liver

An isolated liver perfusion system was used as a simplifying tool to study the metabolism and excretion of benzo(alpha)pyrene (BP) as a prototype carcinogen/mutagen. Phenobarbital (PB) was used to induce liver microsomal enzymes in Sprague-Dawley male rats prior to isolated liver perfusion. Control livers were run simultaneously using generally tritiated (G-3H)BP/BP as substrate in the perfusion medium. Both biliary excretion and liver weight were increased in the induced compared to control liver, but biliary flow when corrected for liver weight is statistically the same for both control and PB-induced livers. The excretion rat of radioactivity in the bile is always higher for PB-induced than for control liver (maximum radioactive excretion at 1 hr). There is a more rapid radioactivity removal in the liver perfusion medium for PB-induced than for control livers. Data are explained by increased metabolism of BP in induced liver leading to the presence of more polar metabolites undergoing preferential biliary excretion than in the control liver. Results support in vivo experimental data. Extracts from liver and bile were tested for microbial mutagenicity by the Ames test (TA 100) after TLC separation. The control liver shows virtually no mutagenicity in bile, only in TLC fractions from the liver. The PB-induced liver shows significant mutagenicity in several TLC fractions in both bile and liver. The net effect of induction is to produce more mutagenic metabolites of BP, excreted in the bile, and presenting a significant exposure of carcinogens/mutagens, and consequent hazard to man.

Metabolic activation and inactivation of chemical carcinogens

Chemical carcinogens are metabolized by numerous pathways catalyzed by enzymes in endoplasmic reticulum and other parts of the cell. Reactions in which functional groups are created (epoxidation and epoxide hydration, catalyzed by cytochrome P-450-linked monooxygenase and epoxide hydratase, respectively) are especially important in the activation of polycyclic hydrocarbon carcinogens to ultimate carcinogenic forms, although other enzymes may also participate in the activation of other chemical carcinogens. Numerous factors, genetic as well as environmental, affect the activities and the balance of different enzymes that participate in carcinogen activation and detoxification. The reasons why carcinogens act on specific target tissues are incompletely understood, although differences in enzyme profiles between tissues certainly contribute to the target tissue variability. Also, the location in the cell (endoplasmic reticulum, nuclear membrane, or other organelle) where the activation takes place is not known. It has been demonstrated that conjugated metabolites of carcinogens may be activated by spontaneous or enzymatic hydrolysis, and this raises the possibility of transport of metabolites to distant target tissues. The concept of metabolic activation of carcinogens by body's own enzymes has led to the development of short-term assay systems, which essentially measure the production of biologically active metabolites from potential carcinogens.