1-Naphthol conjugation in isolated cells from liver, jejunum, ileum, colon and kidney of the guinea pig

Physicochemical and Biological Data for the Development of Predictive Organophosphorus Pesticide QSARs and PBPK/PD Models for Human Risk Assessment

A search of the scientific literature was carried out for physiochemical and biological data [i.e., IC50, LD50, Kp (cm/h) for percutaneous absorption, skin/water and tissue/blood partition coefficients, inhibition ki values, and metabolic parameters such as Vmax and Km] on 31 organophosphorus pesticides (OPs) to support the development of predictive quantitative structure-activity relationship (QSAR) and physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) models for human risk assessment. Except for work on parathion, chlorpyrifos, and isofenphos, very few modeling data were found on the 31 OPs of interest. The available percutaneous absorption, partition coefficients and metabolic parameters were insufficient in number to develop predictive QSAR models. Metabolic kinetic parameters (Vmax, Km) varied according to enzyme source and the manner in which the enzymes were characterized. The metabolic activity of microsomes should be based on the kinetic activity of purified or cDNA-expressed cytochrome P450s (CYPs) and the specific content of each active CYP in tissue microsomes. Similar requirements are needed to assess the activity of tissue A- and B-esterases metabolizing OPs. A limited amount of acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and carboxylesterase (CaE) inhibition and recovery data were found in the literature on the 31 OPs. A program is needed to require the development of physicochemical and biological data to support risk assessment methodologies involving QSAR and PBPK/PD models.

Conjugation of 1-naphthol in primary cell cultures of rat ovarian cells

The present study concerns conjugation of 1-naphthol in primary cultures of rat ovarian cells. Two phase II enzymes catalyzing conjugation, i.e. phenol sulfotransferase (P-SULT) and phenol UDP-glucuronosyltransferase (P-UGT), were measured using 1-naphthol as substrate. The rates of conjugation by the different cell types of the rat ovary were the same at low concentrations and short incubation times. However, after 20 h of incubation the rate of conjugation in cells isolated from ovaries enriched in corpora lutea (CL) exceeded the rate in cells isolated from ovaries enriched in preovulatory follicles. In addition, when the granulosa cells were removed from the preovulatory follicles, the rate of conjugation was 1.7-fold higher, i.e. in the theca/stroma cells. When the cells were incubated with 1-[14C]naphthol and conjugates were subsequently separated by thin-layer chromatography, naphthyl glucuronide was the only conjugate observed. Pentachlorophenol (PCP), a commonly used inhibitor of P-SULT, inhibited 1-naphthol conjugation 50% in cell cultures, as well as in microsomal preparations. alpha-Naphthoflavone (ANF) and ellipticine (ELP), both cytochrome P450 (CYP) inhibitors, affected the conjugation of 1-naphthol in different ways; ANF did not affect P-UGT activity in microsomal preparations, but inhibited 1-naphthol conjugation in cell cultures by as much as 90%. On the other hand, ELP inhibited the conjugation of 1-naphthol up to 99% in the cell cultures, but only 75% in microsomal fractions. Testosterone (TST) and estradiol inhibited this activity approximately equal 50% in both of these experimental systems. Clomiphene citrate (CLF), a drug used to induce ovulation and demonstrating both estrogenic and antiestrogenic effects, did not influence the conjugation of 1-naphthol significantly in the cell cultures. The present findings demonstrate that P-UGT is by far the major enzyme conjugating 1-naphthol in the rat ovary and that commonly used inhibitors of P-SULT and CYPs also inhibit P-UGT activity, either directly or via other mechanisms.

Conjugation of 1-naphthol in human gastric epithelial cells

The biotransformation of xenobiotics is essential to the maintenance of the body's integrity. Mucosal biotransformation has been well documented in the small and large intestine of animals and humans but whether the gastric mucosa plays a role in detoxifying ingested compounds remains largely unknown. The conjugation of the model phenolic compounds, 1-naphthol, by human gastric epithelial cells was assessed in vitro. Freshly isolated and cultured epithelial cells were prepared from surgical specimens obtained from patients undergoing total gastrectomy for cancer. Cell preparations were incubated with 1- 14C-naphthol over 1 hour and the glucuronide and sulphate conjugates formed were separated by thin-layer chromatography. Conjugation of 1-naphthol was observed with both freshly isolated and cultured cells. In freshly isolated cells, the 1 hour turnover of 1 microM 1-naphthol to its glucuronide and sulphate conjugates averaged 19% and 10% respectively. At higher 1-naphthol concentrations, both types of conjugate were formed at about the same rate, up to saturation (apparent Vmax = 0.07 nmol/mg protein/minute, and apparent Km = 40 microM). In cultured cells, the 1 hour turnover of 1 microM 1-naphthol to its glucuronide and sulphate conjugates averaged 35% and 8% respectively. These results suggest that the human gastric mucosa is a detoxifying organ, and that its role with regard to chemical carcinogenesis and drug first pass metabolism deserves further assessment.

The metabolism of benzo(a)pyrene by English sole (Parophrys vetulus): comparison between isolated hepatocytes in vitro and liver in vivo

1. Metabolites and DNA adducts of 3H-benzo(a)pyrene (BaP) formed by isolated hepatocytes from English sole (Parophrys vetulus) in vitro were compared to those in bile and liver of sole exposed i.m. to 3H-BaP. 2. English sole liver was perfused with a collagenase solution and hepatocytes were isolated with greater than 95% viability. Determination of kinetic parameters for metabolism of 3H-BaP showed a Km of 29 +/- 10 microM and an apparent Vmax of 1300 pmol BaP metabolized/10(6) cells per h. 3. Analysis of medium from hepatocyte cultures and bile by ion-pair h.p.l.c. showed significant amounts of radioactivity in regions where glucuronide and glutathione conjugates of BaP metabolites elute. No sulphate conjugates of BaP metabolites were detected. The major unconjugated metabolite formed by hepatocytes was the BaP-9,10-dihydrodiol. 4. Hydrolysis of glucuronide conjugates by beta-glucuronidase and reversed-phase h.p.l.c. analysis of chloroform-soluble metabolites showed the presence of BaP-7,8-dihydrodiol, 1-hydroxyBaP and 3-hydroxyBaP. The identities of these metabolites were confirmed by comparing their fluorescence spectra with those of standard BaP metabolites. 5. Analysis by 32P-postlabelling of the BaP-DNA adducts formed in isolated hepatocytes and liver revealed that major adducts detected are derived from the anti-7,8-dihydrodiol-9,10-epoxideBaP (anti-BaPDE) and syn-BaPDE. 6. Results show that the types of conjugated metabolites and BaP-DNA adducts formed in primary hepatocyte culture were similar to those in bile and liver of English sole exposed to BaP. Thus, isolated hepatocytes from English sole afford a reliable alternative to live fish for studies of the mechanisms of hepatic xenobiotic metabolism and DNA adduct formation in a species shown to be susceptible to induction of hepatocarcinogenesis by PAHs.

Conjugation of 1-naphthol in the gastric mucosa of guinea pigs

1-Naphthol was conjugated in tissue pieces of the gastric wall to naphthol glucuronide (85%) and naphthol sulfate (15%). There was no regioselectivity in different parts of the stomach. In separated gastric mucosal cell populations, the activities of both transferases were highest in the mucous cell fraction (apparent Vmax of glucuronidation: 0.83 nmol/mg protein/min; apparent Vmax of sulfation: 0.11) with only little, or no activity in chief cells and parietal cells. Immunohistochemically glucuronosyltransferase 1 was predominantly localized in surface mucous cells. In conclusion, the gastric mucosa is an important organ for phase II biotransformation.

Impaired sulphation of phenol by the colonic mucosa in quiescent and active ulcerative colitis

Substantial amounts of phenols are produced in the human colon by bacterial fermentation of protein. In the colonic mucosa of animals, phenols are inactivated predominantly by conjugation with sulphate. The purpose of this study was to confirm sulphation of phenols by isolated colonocytes from man and to evaluate mucosal sulphation in inflammatory bowel disease using the phenol, paracetamol, in rectal dialysis bags. The incubation of paracetamol with colonocytes isolated from resected colon specimens (n = 7) yielded a mean (SE) value of 7.0 (0.9) mumols/g dry weight of paracetamol sulphate after 60 minutes but virtually undetectable values of paracetamol glucuronide. Paracetamol sulphate was detected in rectal dialysates from all control subjects, with a mean (SE) value of 4.2 (0.8) nmol/hour. Sulphation was significantly impaired (p less than 0.01) in 19 patients with active ulcerative colitis (0.6 (0.2) nmol/hour) and in 17 patients with ulcerative colitis in remission (1.1 (0.4) nmol/hour). Sulphation in eight patients with Crohn's colitis (4.3 (2.1) nmol/hour) was similar to that in control subjects. Impairment of the capacity of the mucosa to sulphate phenols in quiescent and active ulcerative colitis may pose a metabolic burden on colonic epithelial cells, which are continuously exposed to endogenous phenols from the colonic lumen.

Metabolism of drugs and other xenobiotics in the gut lumen and wall

Metabolism in the gut lumen and wall can decrease the bioavailability and the pharmacological effects of a wide variety of drugs. Bacterial flora in the gut, the environmental pH and oxidative or conjugative enzymes present in the intestinal epithelial cells can all contribute to the process. Bacterial biotransformation is greatest in the colon, while gut wall metabolism is generally highest in the jejunum and decreases distally. Gut wall metabolism may be induced or inhibited by dietary or environmental xenobiotics or by co-administered drugs. Recent evidence suggests that some drugs, food-derived mutagens and other xenobiotics can be metabolized by gut flora and/or gut wall enzymes to reactive species which may cause tumors.

Estimation of phenolic conjugation by colonic mucosa

Conjugation of phenol by the colonic mucosa was assessed in vivo using dialysis tubing containing 1.5 ml of 1 mmol/l acetaminophen (paracetamol) and 10 mmol/l butyrate. These were allowed to equilibrate in the rectum for one hour. The glucuronidated and sulphated conjugates of acetaminophen were measured by high pressure liquid chromatography and bicarbonate concentrations by gas analysis. In 21 subjects without colonic disease sulphate conjugation was observed in all cases, with a mean (SE) of 3.86 (0.66) nmol/hour, while glucuronide conjugation was found in seven of 21 cases. Mean (SE) bicarbonate output of 42.9 (3.9) mumol/hour (n = 21) indicated healthy colonic mucosal metabolism and phenolic sulphation in dialysate and agreed with published sulphation rates obtained with cultured cells of colonic epithelium. Acetaminophen sulphation suggests that the colonic mucosa has an important role in the conjugation of phenols, and the method reported here would be useful in assessing the detoxification capacity of the colonic mucosa in diseases of the rectal mucosa.

Mucosal biotransformation

About 4 million compounds have been described by chemists, and some 60,000 are presently on the market. The search for new chemicals with better properties and less toxicity continues, and future life quality will depend on our ability to find the safest compounds in each field of application. During development of new drugs and chemicals, studies on biotransformation should be done very early, and with adequate analytical tools, in order to get an early understanding of data on bioavailability, metabolic pattern, and toxicity. Though the liver is generally the organ with the highest drug metabolizing activity, it becomes increasingly evident that some extrahepatic organs, such as intestine, kidney, skin, and lung also participate in drug metabolism. The peculiar property of intestinal metabolism is the fact that it modifies chemicals before they enter the circulation. Therefore, an understanding of intestinal metabolism is important for proper interpretation of all pharmacological and toxicological data during development of a new compound. Mucosal biotransformation has recently been reviewed. The present work gives a schematic survey on the topic, shows new trends, and discusses the consequences for toxicological testing of new chemicals.

Localization and characterization of drug-metabolizing enzymes along the villus-crypt surface of the rat small intestine--II. Conjugases

The potential of the epithelial cells of the villus-to-crypt surface of the small intestine of the rat to conjugate xenobiotics was studied. The cells were isolated sequentially in the villus-to-crypt gradient and were found to exhibit heterogeneous distribution patterns and inducer-sensitivities of the conjugating enzymes and their cofactors. UDP-glucuronosyltransferase (GT) activities towards 3-hydroxybenzo[a]pyrene (GT1) and 4-hydroxybiphenyl (GT2) were present in all the cells. The mature upper villus cells were rich in both GT1 and GT2 activities, which declined toward the highly replicating undifferentiated crypt cells. The specific enzyme activities were four times lower in crypt cells than in upper villus cells. The presence of GT1 activity always predominated over GT2 activity. 3-Methylcholanthrene (3-MC) given orally increased GT1 activity by 2-fold in villus cells and about 6-fold in crypt cells, while phenobarbital sodium salt (PB) also markedly induced GT1 of the crypt region. Unlike GT1, GT2 activity was distinctly induced only by PB in all the cells. Both GT1 and GT2 of crypt cells were highly sensitive to inducers, in comparison to the villus cells. The uridine-5-diphosphoglucuronic acid (UDPGA) content ranged from about 0.07 to 0.2 mM in cells from crypts to villus-tip respectively. 3-MC caused a 3-fold increase in UDPGA content in all the cells; PB, however, did not affect UDPGA. The highest glutathione-S-transferase (GST) activity, however, was towards the substrate 1-chloro-2,4-dinitrobenzene; the basal specific enzyme activity varied from about 0.05 to 0.2 mumol per min per mg protein in cells from crypt to upper villus. The enzyme was induced by both types of inducers, being about 2-fold in villus cells and 3- to 5-fold in crypt cells. In contrast, the GSH content was lower in cells with higher GST activity. The endogenous GSH content ranged from 0.8 mM in the upper-villus cells to 3 mM in the crypt cells. The GSH content, however, was not altered by 3-MC or PB treatment of rats. The results demonstrate that xenobiotic conjugation reactions in intestinal cells are much stronger than monooxygenase reactions. The differential and higher sensitivity of the intestinal cells to inducers appears to provide protection to the intestine against xenobiotics during intestinal "first pass".

Role of gut in xenobiotic metabolism

The gastrointestinal tract forms the first line of defense in the body against the main load of xenobiotics. The gastrointestinal mucosa has several mechanisms through which the xenobiotics are modified. The monooxygenase activities in most species are relatively low in the mucosa as compared to the liver, but conjugation, for example, via glucuronide formation proceeds efficiently. UDP-glucuronosyltransferase activities can exceed those in the liver. Glutathione S-transferase activity is also high. The biotransformation activities are readily inducible in the mucosa and this is, at least partly, responsible for the oral-aboral gradient seen in enzyme activities. In rainbow trout glutathione S-transferase is, however, significantly higher at the aboral third than in two oral segments, although in rats the intestinal glutathione S-transferase shows a clear oral-aboral gradient. The gradient is independent of the presence of microflora at least in the case of carboxylesterase and glutathione S-transferase. A similar gradient can also be found from the gut lumen, in both germ-free and specific pathogen-free rats. The cells in the middle of the villi appear to be most responsive under the influence of inducers. The readily occurring induction in the mucosa provides a suitable model for studies on biological effects to defined compounds and mixtures.

Drug metabolism and metabolite transport in the small and large intestine: experiments with 1-naphthol and phenolphthalein by luminal and contraluminal administration in the isolated guinea pig mucosa

The metabolism and metabolite transport of the monophenol 1-naphthol (I) and the diphenol phenolphthalein (II) have been studied in a symmetrical setup of the isolated jejunal and colonic mucosa from the guinea pig (Lauterbach 1977). In both tissues, the main metabolites of I were its sulphoconjugate and glucuronide, but the rate of metabolism, relative proportion of the metabolites and their distribution pattern varied with tissue and drug administration side in the following manner: By luminal administration (50 nmol/ml) in the jejunum, the metabolism was nearly complete within 45 min., more sulphate (1.5-3x) than glucuronide was formed, and both metabolites were predominantly transferred back to the lumen. By blood-side administration, the metabolism was less complete due to a significant decrease of the sulphated fraction. In consequence, more glucuronide (1.5-3x) than sulphate was formed. Moreover, the efflux pattern of the metabolites changed completely; the greater part of the glucuronide fraction now being conveyed to the blood side, whereas the sulphate tended to distribute in a 1:1 fashion on the lumen and blood side. The colonic mucosa behaved in a dissimilar way, since neither I metabolism nor metabolite efflux pattern in this tissue was influenced significantly by drug administration side. More sulphate (1.5-3x) than glucuronide was formed by both routes, and the metabolite distribution was similar to that observed by blood side administration in the jejunum. The changes described above were associated with changes in tissue accumulation of free I and metabolites; accumulation of I by luminal administration in jejunum was insignificant and that of the metabolites small. The main change caused by a higher concentration of I (130 nmol/ml) was a decrease in the sulphated fraction in the colon. II was metabolized at a slower rate than I, and a significant tissue accumulation of free II was observed in all instances. The monoglucuronide was the main metabolite. Only minor amounts of II monosulphate were formed, making its distribution pattern difficult to ascertain. The distribution of II monoglucuronide on the other hand was generally similar to that described for its I analogue.