Studies on hepatic microsomal N- and O-dealkylation of morphine analogues.
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Animals↗
Biomedical subjects
T R Tephly
Publications and source records attributed to T R Tephly.
Reduction of hepatic tetrahydrofolate and inhibition of exhalation of 14CO2 formed from [dimethylamino-14C]aminopyrine in nitrous oxide-treated rats.
The exhalation of 14CO2 after the administration of [dimethylamino-14C]aminopyrine to an organism is assumed to reflect the demethylation of aminopyrine by hepatic mixed-function oxidase activity. The formaldehyde formed as a result of the demethylation of aminopyrine is then sequentially oxidized to formic acid and CO2. The last step in the pathway, i.e., formate oxidation, is dependent upon tetrahydrofolate; thus, factors which alter hepatic tetrahydrofolate potentially may modify 14C-aminopyrine metabolism to 14CO2 in vivo. Exposure of rats to nitrous oxide (N2O) produces a significant reduction in hepatic tetrahydrofolate as a result of the inhibition of 5-methyltetrahydrofolate:homocysteine methyltransferase activity (E.C. 2.1.1.13). In the present study, exposure of rats to N2O/O2 (1:1) for 4 hr prior to the administration of 14C-aminopyrine (40 or 400 mumoles per kg) produced a 60% reduction in the peak rate of 14CO2 exhalation and a 45% decrease in the total 14CO2 exhaled within 2 hr. In control experiments, exposure of rats to nitrogen/O2 (1:1) produced no effect on 14C-aminopyrine metabolism to 14CO2. Administration of methionine (1.3 mmoles per kg) 30 min prior to 14C-aminopyrine administration reversed the inhibition of 14CO2 exhalation and reduction in hepatic tetrahydrofolate observed in N2O-exposed animals. Aminopyrine (400 mumoles per kg) administration to air-breathing rats did not affect the level of urinary formate, but exposure to N2O produced a 40-fold increase. Aminopyrine administration to N2O-exposed rats produced a 75% increase in urinary formate as compared to rats treated with N2O alone.(ABSTRACT TRUNCATED AT 250 WORDS)
Aminopyrine↗
Effect of side chain length on bile acid conjugation: glucuronidation, sulfation and coenzyme A formation of nor-bile acids and their natural C24 homologs by human and rat liver fractions.
The effect of side chain length on bile acid conjugation by human and rat liver fractions was examined. The rate of conjugation with glucuronic acid, sulfate and coenzyme A of several natural (C24) bile acids was compared with that of their corresponding nor-bile acids. The rate of coenzyme A ester formation by nor-bile acids was much lower than that of the natural bile acids. In human liver microsomes, the rate of coenzyme A formation was less than 8% of the rate for the corresponding C24 bile acid. Rat liver microsomes formed the coenzyme A ester of nor-bile acids less than 20% of the rate of their corresponding C24 homologs. Glucuronidation rates were greater than sulfation rates in both species. With human liver microsomes, nor-bile acids were glucuronidated more rapidly than their corresponding C24 homologs, whereas with rat liver microsomes the reverse was true. Purified 3 alpha-OH androgen UDP-glucuronyltransferase catalyzed the glucuronidation of both nor-bile acids and bile acids. Human liver cytosol sulfated nor-bile acids more slowly than the corresponding bile acids. Rat liver cytosol, however, sulfated nor-bile acids more rapidly than the corresponding bile acids. The highest rate was seen with lithocholylglycine. The results indicate that the novel biotransformation of nor-bile acids seen in vivo--sulfation and glucuronidation rather than amidation--is most likely explained as a consequent of defective amidation, to which the rate of coenzyme A formation contributes. Thus, side chain and nuclear structures as well as species differences in conjugating enzyme activity are determinants of the pattern of bile acid biotransformation by the mammalian liver.
Animals↗
Differential glucuronidation of bile acids, androgens and estrogens by human UGT1A3 and 2B7.
In this work, UDP-glucuronosyltransferases (UGTs), UGT1A3, 2B7(H268) and 2B7(Y268), stably expressed in human embryonic kidney cells (HK293) were used to assess glucuronidation activities with a variety of steroid hormone and bile acid substrates. The rate of synthesis of carboxyl- and hydroxyl-linked glucuronides was determined under optimal reaction conditions. Expressed UGT1A3 catalyzed bile acid glucuronidation at high rates exclusively at the carboxyl moiety for all compounds tested. In contrast, UGT1A4 catalyzed bile acid glucuronidation at very low rates exclusively at the 3alpha-hydroxyl function. Both UGT2B7 allelic variants glucuronidated the bile acid substrates at both carboxyl and hydroxyl moieties, however, the 3alpha-hydroxyl position was preferentially conjugated compared to the carboxyl function. Similarly, androsterone, a 3alpha-hydroxylated androgenic steroid, was glucuronidated at very high rates by expressed UGT2B7. Of the estrogenic compounds tested, UGT2B7 catalyzed the glucuronidation of estriol at rates comparable to those determined for androsterone. Other structural discrimination was found with UGT2B7 which had activity toward estriol and estradiol exclusively at the 17beta-OH position, yielding the cholestatic steroid D-ring glucuronides.
Androgens↗
Isolation and purification of UDP-glucuronosyltransferases.
UDPGTs are members of a class of enzymes located in the endoplasmic reticulum and are encoded by a multigene family. These proteins are responsible for the glucuronidation of hundreds of xenobiotics of many chemical classes and many endogenous substances such as steroid hormones, bile acids, and bilirubin. There are a number of UDPGTs which have been identified by purification and characterization studies and a significant number which have been characterized by expression of cDNAs. On the basis of the primary structures elucidated they appear to have marked similarities (5) and are highly conserved. However, key differences in their functional properties appear to depend primarily on differences in amino acid sequences at or about the NH2-terminal area of the protein (5). Many of the UDPGTs have an extraordinarily broad substrate specificity; a few, however, are relatively specific for a given class of substrate (morphine, DT-1 UDPGTs). This places a burden on investigators to clearly identify which substrate and how many UDPGTs will be involved in any analysis of rates of glucuronidation in microsomal preparations. Caution should also be advised for extrapolation of data from hepatic microsomes of experimental animals to human hepatic microsomal preparations because human liver microsomes possess UDPGTs which are qualitatively different and, in certain cases, UDPGTs are present in human liver which are not present in lower animals.
Animals↗
Studies on tertiary amine UDP-glucuronosyltransferases from human and rabbit hepatic microsomes.
The conversion of tertiary amines to quaternary ammonium glucuronides was investigated in human liver microsomes, and characteristics of the UDP-glucuronosyltransferase (UGT) catalyzing quaternary ammonium glucuronidation were evaluated. In addition, a rabbit liver microsomal UGT mediating this reaction was studied. The kinetics of quaternary ammonium glucuronidation of cyproheptadine, tripelennamine, amitriptyline, and doxepin in intact human liver microsomes was determined. Tripelennamine was found to have the lowest apparent KM and was used as a representative substrate for further studies. A polyclonal antibody preparation raised in sheep against rabbit liver p-nitrophenol UGT was found to inhibit tripelennamine glucuronidation in solubilized human liver microsomes, but had no effect on p-nitrophenol, 4-methylumbelliferone, 4-aminobiphenyl, estriol, morphine, or naloxone glucuronidation. This antibody also inhibited tripelennamine glucuronidation in solubilized rabbit liver microsomes, but had little or no effect on estrone, testosterone, estradiol, androsterone, and morphine glucuronidation. Chlorpromazine competitively inhibited tripelennamine glucuronidation. This inhibition was markedly enhanced by UV light irradiation. [3H] Chlorpromazine binding to solubilized human liver microsomes was also increased by UV light. The binding was antagonized by substrates for tertiary amine UGT but not by substrates for morphine UGT. These studies suggest that the tertiary amine UGT is photo-affinity-labeled by chlorpromazine. Furthermore, it would appear from immunoinhibition and [3H]chlorpromazine labeling experiments that tertiary ammonium glucuronidation is catalyzed by a unique and distinct UGT in rabbit and human liver microsomes.
Amines↗
Glucuronidation of monohydroxylated short chain bile acids by human liver microsomes and purified human liver UDP-glucuronosyltransferases.
The glucuronidation of monohydroxylated bile acids and their analogs with a shortened side chain (short-chain bile acids) by human liver microsomes and by two UDP-glucuronosyltransferases purified therefrom has been studied in vitro. In microsomes, all 18 substrates tested underwent glucuronidation; the rate of reaction and the site of attachment of glucuronic acid (hydroxyl group in position 3, side chain carboxyl group, or various ratios of both products) were strongly dependent on the length of the side chain, the configuration of the 3-hydroxyl group, and the configuration of the A/B ring junction (5 alpha-H/5 beta-H). Two UDP-glucuronosyltransferases (UDPGTs) purified from human microsomes, designated "pl 7.4" and "pl 6.2" according to their behavior in chromatofocusing, accounted for the formation of hydroxyl-linked glucuronides of a different pair of bile acids each. The pl 7.4 human liver UDPGT catalyzed the glucuronidation of C20 and C22 with the 3 alpha-OH, 5 beta-H configuration, while the pl 6.2 human liver UDPGT catalyzed the glucuronidation of either 3-OH epimer of C21 and C24 acids with the 5 alpha-H configuration. The enzymes displayed a relatively high selectivity in that they did not accept any of the remaining 14 bile acids as substrates; none of the enzymes led to the formation of a carboxyl-linked glucuronide. In addition, purified human liver UDPGT did not catalyze the glucuronidation of cholate, deoxycholate, chenodeoxycholate, ursodeoxycholate, lithocholate, hyocholate, hyodeoxycholate, bilirubin, morphine, or 4-hydroxybiphenyl. The above results suggest that several bile-acid UDP-glucuronosyltransferases of high specificity exist in human liver.
Bile Acids and Salts↗
Morphine metabolism. V. Isolation of separate glucuronyltransferase activities for morphine and p-nitrophenol from rabbit liver microsomes.
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Animals↗
Separation, purification, and characterization of digitoxigenin-monodigitoxoside UDP-glucuronosyltransferase activity.
Glucuronidation of digitoxigenin-monodigitoxoside was investigated in liver microsomes from spironolactone-induced male Wistar rats. Isolation of a specific digitoxigenin-monodigitoxoside UDP-glucuronosyltransferase was possible utilizing chromatofocusing chromatography with a gradient from pH 10.1 to 8.0 after solubilizing the microsomal protein with the nonionic detergent Emulgen 911. The digitoxigenin-monodigitoxoside UDP-glucuronosyltransferase was further purified using UDP-hexanolamine Sepharose 4B affinity chromatography. The highly purified (75-fold) enzyme showed activity toward digitoxigenin-monodigitoxoside and slight activity toward digitoxigenin-bisdigitoxoside, whereas digitoxin and substrates for p-nitrophenol, 17 beta-OH steroid, and 3 alpha-OH steroid UDP-glucuronosyltransferases were not glucuronidated. In addition, bilirubin, morphine, estrone, 4-hydroxybiphenyl, and aromatic amines were not glucuronidated by this protein. These results strongly confirm the presence of a form of UDP-glucuronosyltransferase, which is highly specific for the glucuronidation of digitoxigenin-monodigitoxoside.
Animals↗
The regulation of heme synthesis related to drug metabolism.
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Aminobenzoates↗
Morphine metabolism. I. Evidence for separate enzymes in the glucuronidation of morphine and p-nitrophenol by rat hepatic microsomes.
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Age Factors↗
Morphine metabolism. II. Studies on morphine glucuronyltransferase activity in intestinal microsomes of rats.
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Animals↗
A procedure for the rapid separation and purification of UDP-glucuronosyltransferases from rabbit liver microsomes.
A technique has been developed which rapidly separates and purifies UDP-glucuronosyltransferases from liver microsomes of untreated rabbits. by use of this method, highly purified estrone and p-nitrophenol UDP-glucuronosyltransferases can be obtained in good yield in about 48 hr. Microsomes were solubilized with the nonionic detergent Emulgen 911 in low ionic strength buffers and applied to a DEAE-cellulose column equilibrated with low ionic strength buffers. UDP-glucuronosyltransferase activities were then eluted in a stepwise fashion with increasing concentration of KCl. Three fractions were studied. The first two fractions contained only estrone UDP-glucuronosyltransferase activity while a third contained p-nitrophenol UDP-glucuronosyltransferase activity. Each fraction was directly applied to a UDP-hexanolamine Sepharose-4B column, which was then washed extensively with KCl, and the transferases were eluted with UDP-glucuronic acid. A method for separating the transferases on the affinity column is presented. Testosterone and morphine could not be conjugated by any of the purified enzymes.
Animals↗
Steroid 3- and 17-OH UDP-glucuronosyltransferase activities in rat and rabbit liver microsomes.
Steroid glucuronidation was investigated in solubilized female rat and rabbit liver microsomes and in preparations of UDP-glucuronsyltransferase (UDPGT) activity resolved from these organelles. Solubilized rabbit liver microsomes possessed relatively high UDPGT activity towards estrone and beta-estradiol but not testosterone. Glucuronidation observed at the 3-OH position of beta-estradiol is 20-fold greater than at the 17-OH position. Estrone UDPGT activity, highly purified from rabbit liver microsomes, was active towards estrone and the 3-OH position of beta-estradiol but not towards testosterone (17-OH) or the 17-OH position of beta-estradiol. Thus, estrone UDPGT activity demonstrated essentially the same specificity for conjugation of the 3-OH position of beta-estradiol as observed in microsomes. Solubilized liver microsomes from female rats possessed approximately 4-fold more activity towards testosterone (17-OH) than estrone (3-OH). Rat liver microsomes formed about 2.5-fold more beta-estradiol 17-glucuronide than 3-glucuronide. Following a Chromatofocusing procedure where a gradient from pH 9-7 was employed, an eluant fraction was obtained that was enriched in estrone UDPGT activity relative to testosterone UDPGT activity. The fraction preferentially conjugated the 3-OH position of beta-estradiol. Two eluant fractions were obtained which demonstrated high levels of UDPGT activity towards testosterone and beta-estradiol but not estrone. Both fractions possessed a UDPGT activity that displayed a high degree of specificity for conjugation of the 17-OH position of beta-estradiol. A 17-OH steroid UDPGT was purified to apparent homogeneity from one of these fractions. These results suggest that separate UDPGT activity exists in female rat liver for the glucuronidation of the 3- and 17-OH positions of beta-estradiol. In female rabbit liver, beta-estradiol is predominantly conjugated at the 3-OH position by a single form of UDPGT.
Animals↗
Studies on the glucuronidation of dopamine D-1 receptor antagonists, SCH 39166 and SCH 23390, by human liver microsomes.
Dopamine D-1 receptor antagonists are currently under investigation for use as antipsychotic agents. Two potent and selective D-1 receptor antagonists, SCH 39166 and SCH 23390, have been studied extensively in various experimental animal models. SCH 39166 has a more prolonged duration of action in primates in vivo and a lower rate of in vitro glucuronidation by microsomes from squirrel monkey liver. Because the rate of glucuronidation seems to govern the duration of action and may limit the use of these agents in humans, the glucuronidation of SCH 39166 and SCH 23390 by microsomes isolated from human liver was studied. The rates of glucuronide formation (Vmax) for SCH 39166 were much lower than those of SCH 23390, yet the KM values were similar. Therefore, the average efficiency (Vmax/KM) of SCH 39166 glucuronidation was only 14% that of SCH 23390. These results agree with previous studies in hepatic microsomes from squirrel monkeys. Marked inhibition of SCH 39166 glucuronidation by SCH 23390 and its pharmacologically inactive stereoisomer, SCH 23388, was observed. The inactive stereoisomer of SCH 39166, SCH 39165, was a weak inhibitor. In contrast, substrates for morphine UDP-glucuronosyltransferase (UGT), and p-nitrophenol, an alternative substrate for numerous human hepatic UGTs, did not inhibit SCH 39166 glucuronidation. Further separation of human hepatic UGTs activities using chromatofocusing chromatography indicated that SCH 39166 UGT activity was distinct from human hepatic UGT2B15 and human hepatic pI 6.2 UGT activity. Thus, a unique human hepatic UGT may be involved in SCH 39166 glucuronidation.(ABSTRACT TRUNCATED AT 250 WORDS)
Benzazepines↗
Stable expression of a human liver UDP-glucuronosyltransferase (UGT2B15) with activity toward steroid and xenobiotic substrates.
A full-length cDNA clone (HE8a) for a human hepatic UDP-glucuronosyltransferase was isolated from a human liver cDNA library and stably expressed in human embryonic kidney 293 (HK293) cells. Sequence analysis of the cDNA revealed that it was identical to UDPGTh-3 isolated by Chen et al. (Biochemistry 32, 10648-10657, 1993). HE8a is a member of the UGT2B gene family, and it has been designated UGT2B15. Over 100 compounds were tested for their reactivity with the expressed protein. UGT2B15 stably expressed in HK293 cells displayed glucuronidation activity toward several classes of xenobiotic substrates, including simple phenolic compounds, 7-hydroxylated coumarins, flavonoids, anthraquinones, and certain drugs and their hydroxylated metabolites. In addition, the expressed enzyme also catalyzed the glucuronidation of endogenous estrogens and androgens. Apparent KM and enzyme efficiency values for certain food-derived substrates (e.g., naringenin and eugenol) for expressed UGT2B15 were similar to those determined for endobiotic substrates, suggesting that some naturally occurring substances are good substrates for this enzyme and that glucuronidation of endogenous compounds could be affected by xenobiotics derived from dietary sources.
Cells, Cultured↗