A comparison of levels of glutathione transferases, cytochromes P450 and acetyltransferases in human livers

The Therapeutic Use of Acetaminophen in Patients with Liver Disease

Acetaminophen has been used safely and effectively for many years to manage pain and/or fever in patients of all ages. It is commonly recommended as first-line therapy for a variety of patients and conditions, including the elderly, children with viral illnesses, and patients with osteoarthritis, gastrointestinal conditions, bleeding disorders, cardiovascular disease, or renal disease. However, its use is often avoided in patients with chronic liver disease. The perception that acetaminophen should be avoided in such patients arose from awareness of the association between massive acetaminophen overdose and hepatotoxicity, combined with a lack of understanding of the metabolism of acetaminophen in patients with liver disease. There are various theoretical mechanisms of acetaminophen hepatotoxicity in chronic liver disease including: altered metabolism and depleted glutathione stores that would be expected to increase accumulation of the hepatotoxic intermediate, N-acetyl-p-benzoquinone imine (NAPQI). Available studies in patients with chronic liver disease, however, have shown that although the half-life of acetaminophen may be prolonged, cytochrome P-450 activity is not increased and glutathione stores are not depleted to critical levels in those taking recommended doses. Furthermore, acetaminophen has been studied in a variety of liver diseases without evidence of increased risk of hepatotoxicity at currently recommended doses. Therefore, acetaminophen can be used safely in patients with liver disease and is a preferred analgesic/antipyretic because of the absence of the platelet impairment, gastrointestinal toxicity, and nephrotoxicity associated with nonsteroidal antiinflammatory drugs.

Glutathione conjugation of perchloroethene in subcellular fractions from rodent and human liver and kidney

Perchloroethene (Per) is a widely used industrial solvent and common environmental contaminant. In rats, long-term inhalation of Per is known to cause a small increase in the incidence of renal tubule cell tumors in males only; renal toxicity is seen in female rats and in both sexes of mice after prolonged Per exposure. The renal toxicity of Per is likely mediated by a glutathione-dependent bioactivation reaction. Glutathione S-transferase mediated formation of S-(1,2,2-trichlorovinyl)glutathione is the first step in a sequence of reactions finally resulting in the formation of reactive intermediates in the kidney. In this study, we compared the enzymatic rates of formation of S-(1,2,2-trichlorovinyl)glutathione in liver and kidney subcellular fractions from rats, mice, and from both sexes of humans (n = 11). In microsomal fractions from the liver and kidney of all three species, enzymatic formation of S-(1,2,2-trichlorovinyl)glutathione from Per could not be observed. S-(1,2,2-Trichlorovinyl)glutathione formation (the structure was confirmed by electrospray mass spectrometry) was observed in liver cytosol from both male and female rats and mice. However, the rates of S-(1,2,2-trichlorovinyl)glutathione formation in liver cytosol from male rats (84.5+/-12 pmol/mg per min) were approximately four times higher than from female rats (19.5+/-8 pmol/mg per min) and from both sexes of mice (27.9+/-6 and 26.0+/-4 pmol/mg per min). Low rates of S-(1,2,2-trichlorovinyl)glutathione formation were also seen in kidney cytosol from mice (12+/-6 pmol/mg per min), but not from rats. In human liver subcellular fractions, enzymatic formation of S-(1,2,2-trichlorovinyl)glutathione could not be detected. The human liver cytosolic fractions, however, exhibited glutathione S-transferase activity (as determined using 1-chloro-2,4-dinitrobenzene and hexachlorobutadiene as marker substrates) in the same order of magnitude as rat and mouse liver cytosol. In contrast to other marker activities for glutathione S-transferases, the ability of all human liver cytosol samples to catalyze the glutathione conjugation of 1,2-dichloro-4-nitrobenzene was three orders of magnitude lower compared to rat and mouse liver cytosol. 1,2-Dichloro-4-nitrobenzene conjugation was also four times higher in liver cytosol from male rats compared to female rats. The results suggest that the ability of the human liver to catalyze the formation of S-(1,2,2-trichlorovinyl)glutathione from Per is at least two orders of magnitude lower than that of rat liver, and that sex-specific differences in the extent of hepatic conjugation of Per with glutathione, which may be relevant for nephrotoxicity, occur in rats.

Human glutathione transferase catalysis of the formation of S-nitrosoglutathione from organic nitrites plus glutathione

The kinetics of spontaneous and human glutathione transferase catalysed formation of S-nitrosoglutathione (GSNO) from glutathione (GSH) and n-butyl- or amyl nitrite have been studied. At physiological pH and temperature, k2 values of 22.3 and 21.0 M-1.min-1 were obtained for n-butyl- and amyl nitrites, respectively. Rate enhancements, (kcat/Km x k2) x 10(-4), due to purified human GSH transferases A1-1, A2-2 and M1a-1a were, respectively, 7.00, 2.94 and 10.6 for n-butyl nitrite and 121, 3.92 and 34.5 for amyl nitrite. GSH transferase P1-1 showed no detectable catalysis of the formation of GSNO. The data suggest that the presence of GSTs A1-1, A2-2 or M1-1 contribute substantially to intracellular metabolism of alkyl nitrites to GSNO. The results may be significant with regard to the immunotoxicity of alkyl nitrites.

The expression and environmental regulation of P450 enzymes in human placenta

The human placenta oxidizes several xenobiotics, although the spectrum of substrates and metabolic activities when compared with the liver appears somewhat restricted. Maternal cigarette smoking or PCB exposure increases the expression of CYP1A1. This induced activity is able to catalyze the activation of benzo(a)pyrene (B(a)P) into DNA-bound products, both in vitro and in vivo. Studies on adult human liver bolster the concept that CYP1A1 and -1A2 are differentially expressed in hepatic and extrahepatic tissues. Studies with cDNA probe or enzyme specific antibodies and substrates for CYP2A, -2B, -2C, -2D, and -2E gene products have yielded negative results. There are only minimal activities that can be found in substantial quantities in placentas without maternal smoking; one example is 7-ethoxycoumarin O-deethylase (ECOD). Aromatase and cholesterol side-chain cleaving P450 mRNAs, proteins, and activities are measurable in human placentas and do not seem to be affected by maternal cigarette smoking.

Viability and function in primary culture of adult hepatocytes from various animal species and human beings after cryopreservation

Cryopreserved hepatocytes from various animal species and human beings were tested for their ability to survive and function in primary culture. The freeze/thaw protocol primarily designed for rat hepatocytes was used with slight modifications for the cells of all other species; it consisted of suspending parenchymal cells in the Leibovitz L15 medium containing 10% fetal calf serum and 10% to 16% dimethyl sulfoxide. After transient storage at 4 degrees C cell suspensions were transferred to -20 degrees C and then to -70 degrees C before being plunged in liquid nitrogen. Hepatocytes were stored for a few weeks to 4 yr. Prolonged storage did not augment loss of cell viability and function. Cell viability after thawing was estimated by the trypan blue exclusion test, and attachment efficiency to plastic was estimated by measuring intracellular lactate dehydrogenase content. Similar values were obtained for most species tested; after cryopreservation cell viability and attachment were decreased by 10% to 25% and by 40% to 50%, respectively. A lower attachment rate was found with dog hepatocytes. Total cytochrome P-450 and protein synthesis were compared in fresh and cryopreserved cells from four species after 4, 24, 48 or 72 hr of culture. Similar values were found in both cells after 24 or 48 hr of culture. In addition, drug-metabolizing activities were measured in human hepatocytes from five donors. In most cases phenacetin deethylation activity was decreased whereas procainamide N-acetylation and paracetamol sulfoconjugation and glucuronidation were increased in cryopreserved cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The human glutathione S-transferase supergene family, its polymorphism, and its effects on susceptibility to lung cancer

Cytosolic glutathione S-transferases (GSTs) are a supergene family of dimeric enzymes capable of detoxifying a number of carcinogenic electrophiles. Of the numerous components of tobacco smoke, the polycyclic aromatic hydrocarbons appear to be the principal compounds that yield substrates for these enzymes, GSTM1-1 being effective with those PAH derivatives so far studied; however, the gene locus for GSTM1 is polymorphic, containing two well-characterized expressing genes and a null allele. Use of cDNA for GSTM1-1 or appropriate fragments of genomic clones as probes in Southern blots indicated that the null allele is due to the absence of GSTM1. In preliminary experiments, described here, with lung tissue from smokers, levels of 32P-postlabeled nuclease P1-enhanced DNA adducts were inversely correlated with levels of antigen cross-reacting with antibody to GSTM1-1, suggesting that initiation depends on the expression of GSTM1-1. Since similar quantities of DNA adducts and GSTM1-1 activity have been shown to occur in bronchial and peripheral lung, however, the development of malignancy, which is usually in the bronchial region, presumably depends on additional factors that bring about promotion and progression, which are not necessarily affected by GSTM1 expression. Two epidemiological studies have been carried out in which a possible correlation between the absence of GSTM1 and lung cancer incidence is considered. In the first, involving a U.S. population sample, smokers with and without lung cancer were phenotyped, and a highly significant correlation between the absence of GSTM1-1 activity and adenocarcinoma of the lung was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme induction by drugs and toxins

Enzyme induction by drugs mostly concerns those enzymes involved in drug metabolism: cytochromes P-450, UDP-glucuronosyltransferases, glutathione S-transferases, gamma-glutamyltransferases and epoxide hydrolases. A large variety of molecular forms exists, but not all of them are inducible (e.g. the inducible cytochromes P-450 in man are members of family IA, IIA, IIC, IIE, IIIA). Induction is most common in the liver, but also occurs in other organs (lung, placenta, lymphocytes). Over the past 20 years a relatively small number of drugs and environmental chemicals have been identified as enzyme inducers, perhaps fewer than early studies suggested. Information on inducing properties must be obtained as early as possible during the development of a new drug and made available to clinicians and clinical chemists when the drug is marketed. The main consequences of enzyme induction are changes in pharmacokinetics of the drug itself or of an associated drug. Much progress has been made in methods to identify these inducers.