An enzyme from rat liver catalysing conjugations with glutathione

Glutathione conjugation of some xenobiotics by Ascaris suum and Moniezia expansa

1. The cestode Moniezia expansa and the nematode Ascaris suum both possess enzymes catalysing the conjugation of glutathione (GSH) with 1-chloro-2,4-dinitrobenzene. 2. The GSH-S-aryl transferase (GSH-S-transferase A) was present in the cytosol of the cestode proglottids and of the nematode intestinal epithelial cells. Other tissues did not contain measurable activity. The enzymes from both species had mol. wt. of about 37 000 and broad pH optima around pH 8.3. Both enzymes were inhibited by Cu2+, Fe3+ and Hg2+ at 1 mM and stimulated by Co2+. 3. Neither M. expansa nor A. suum possessed measurable DDT dehydrochlorinase activity. GSH-S-epoxide transferase (GSH-S-transferase E) activity was indicated in both species; neither species effected the conjugation of bromo- or chlorobenzene. 4. Halogenated anthelmintics were not metabolized to GSH conjugates in the helminths studied and did not inhibit GSH-S-aryltransferase activity towards chlorodinitrobenzene.

The preparation of ligandin with glutathione-S-transferase activity from porcine liver cytosol affinity chromatography on bromosulphophthalein-Sepharose

A simple and rapid method for the purification of porcine ligandin with glutathione-S-transferase activity is presented. After ion-exchange chromatography on DEAE-Sephadex, ligandin is isolated from porcine liver cytosol by affinity chromatography on bromosulphophthalein-Sepharose and gel filtration of Sephadex G-100. Evidence is presented that the purified ligandin is homogeneous with respect to polyacrylamide-gel electrophoresis (7.5%) and sodium dodecylsulphate-gel electrophoresis. Physico-chemical investigations show that the purified ligandin has properties similar to those of ligandin isolated from other species with respect to molecular weight, amino-acid composition, secondary structure and catalytic activity. As is the case for human and rat ligandin, porcine ligandin binds bilirubin. Evidence is also presented that porcine liver cytosol contains several bromosulphophthalein-binding proteins with basic isoelectric points lacking catalytic activity.

Biodegradation of alpha-hexachlorocyclohexane. VII. Resolution, purification, and characterization of an alpha-HCH dechlorinating enzyme from rat liver cytosol

From rat liver cytosol, an enzyme was isolated which catalyzes the dechlorination of alpha-, gamma-, and delta-HCH. The enzyme also catalyzes the conjugation of GSH with 1,2-dichloro-4-nitrobenzene, and with 1 -chloro-2,4-dinitrobenzene. The enzyme has a molecular weight of about 46000 and its molecule is composed of two subunits of similar size. The optimum for the dechlorination of alpha-HCH lies at pH 8.0. The Michaelis constant is 0.12 mM for alpha-HCH (with 1 mM GSH as constant substrate), and maximal velocity was determined to be 0.25 moles Cl- -min -1 per mole enzyme.

Comparative metabolic studies of phenacetin and structurally-related compounds in the rat

1. A comparative study of the metabolism of [acetyl-14C]phenacetin, [acetyl-14C]methacetin, [acetyl-14C]paracetamol and [acetyl=14C]acetanilide in the rat is reported. 2. The extent of N-deacetylation, evidenced by the measurement of respired 14CO2, varied, being greatest with acetanilide (25-31%) and least with paracetamol (6%). 3. The major urinary metabolites in each case were N-acetyl-p-aminophenyl sulphate and N-acetyl-p-aminophenyl glucuronide; the relative proportions varied with the sex of the animals and as a result of extended dosage. 4. The metabolism of [ethyl-14C]phenacetin and [ethyl-14C]phenetidine was investigated and the extent of O-dealkylation determined by measurement of respired 14CO2. 5. The metabolic pathways of some related glycolanilides and oxanilic acids included N-deacylation, and in the glycolanilides, oxidation of the glycollic group.

Evidence for the possible formation of a toxic tyrosine metabolite by the liver microsomal drug metabolizing system

The toxicity of tyrosine in the mouse was found to be affected by agents which interact with the liver microsomal drug metabolizing system. Pretreatment of mice with SKF 525A or cobaltous chloride, two different types of inhibitors of the drug metabolizing system, afforded a marked protection against tyrosine toxicity. Pretreatment with phenobarbital or 3-methylcholanthrene, two inducers of the drug metabolizing system, potentiated the toxic effects of tyrosine. Toxicity, as determined by lethality, was also found to be correlated with a depletion of liver glutathione levels. It is concluded that the liver microsomal drug metabolizing system is capable of converting tyrosine to a toxic metabolite, conceivably a 2.3 epoxide of tyrosine. The present results are discussed in relation to the human disease tyrosinemia (tyrosinosis).

Drug induction and sex differences of renal glutathione S-transferases in the rat

Treatment of male rats with 3,4-benzopyrene, 3-methylcholanthrene and phenobarbital resulted in the induction of glutathione S-aryl- and S-aralkyl-transferase activities in kidney cytosol. Benzopyrene produced 77 and 44% increases in aryl and aralkyl activities respectively. Methylcholanthrene caused 73 and 86% increases in the retrospective activities, whereas phenobarbital treatment increased only aralkyl activity (51%). There was no effect on epoxide or alkyl glutathione S-transferase activities with these treatments. Differences were found between the specific activities of the four glutathione S-transferases in females and males, with the following female/male ratios: aryl 0.74; aralkyl 2.37; epoxide 1.52; alkyl 1.33. No changes in Km values were observed relative to drug induction or sex differences. Comparisons are made between the findings of this report and corresponding experiements with liver.

Purification and characterization of two glutathione S-aryltransferase activities from rat liver

Two forms of glutathione S-aryltransferase were purified from rat liver. The only differences noted between the two forms were in the chromatographic and electrophoretic properties, which permitted the separation of the two species. The molecular weights of the enzyme and its subunits were estimated as about 50000 and 23000 respectively. The steady-state kinetics did no follow Michaelis-Menten kinetics when one substrate concentration was kept constant while the second substrate concentration was varied. Several S-substituted GSH derivatives were tested as inhibitors of the enzymic reaction. The enzyme was inactivated by thiol-group reagents.

The influence of environmental agents on prostaglandin biosynthesis and metabolism in the lung. Inhibition of lung 15-hydroxyprostaglandin dehydrogenase by exposure of guinea pigs to 100 per cent oxygen at atmospheric pressure

Enzymes in the 100 000g supernatant fraction of guinea-pig lungs, in the presence of NAD-+, converted PGF-2 alpha (prostaglanding F-2 alpha) into a less-polar compound. The u.v. spectrum of this metabolite showed a strong absorption band at 230 nm, which is characteristic of a carbonyl group in conjugation with a double bond. Reduction of this metabolite with NaBH4 resulted in a compound that behaved like PGF2 ALPHA on t.l.c. and g.l.c. From this evidence we concluded that PGF2alpha is metabolized in vitro to 15-oxo-PGF2 alpha by the NAD-+-dependent prostaglandin dehydrogenase system of guinea-pig lung. The effect of exposure of the animal to SO-2 and O2 on the rate of prostaglanding biosynthesis and catabolism by lung fractions in vitro was studied. Exposure of guinea pigs to 500 p.m. of SO2 for 5h or to 50p.p.m for 9 days (6h/day) did not alter the production or degradation of prostaglandings by lung fractions in vitro. In contrast, exposure of guinea pigs to 100% O2 for 48 h inhibited the rate of prostaglanding metabolism in vitro by 60-70% without significantly altering the rate of biosynthesis by lung fractions. Inhibition of prostaglandin dehydrogenase activity in vitro by lung fractions after exposure of the animal to O2 was dependent on the duration of exposure. Gluthathione S-aryltransferase and catechol O-methyltransferase activites of guinea-pig lung 100 000g supernatant were unaltered by exposure of the animal to O2. Thus it appears that inhibition of pulmonary prostaglandin dehydrogenase by exposure of the animal to O2 is not the result of a general toxic response. It was postulated that the inhibition of prostaglanding dehydrogenase may occur after exposure of the animal to other oxidant gases.

Drug induction of hepatic glutathione S-transferases in male and female rats*

The induction of the glutathione S-transferases by phenobarbital and polycyclic hydrocarbons was studied in male and female rats. Administration of phenobarbital resulted in 60-80% increase in S-aryl and S-aralkyl enzyme specific activities, whereas the S-epoxide and S-alkyl activities were increased by 30-40%. In following the sequence of induction, the former two activities were noted to reach peak activities before an increase in the latter two activities was observed. Both 3-methylcholanthrene and 3,4-benzopyrene were shown toi nduce these four enzymic activities, although without the discrimination between pairs of activities noted with phenobarbital. No change in Km accompanied the increase in Vmax. after induction by drugs, and no change occurred in Ki for sulphobromophthalein inhibition. Significantly lower enzyme specific activities were found for three of the activities studied in female rats but no difference was observed in the S-alkyltransferase activity. However, the proportional increase in the enzymic activities in response to phenobarbital was the same in males and females. These studies demonstrate the drug induction of a group of cytosolic drug-metabolizing enzymes as well as the identification of sex differences in these activities.

Cross specificity in some vertebrate and insect glutathione-transferases with methyl parathion (dimethyl p-nitrophenyl phosphorothionate), 1-chloro-2,4-dinitro-benzene and s-crotonyl-N-acetylcysteamine as substrates

1. Enzymes catalysing the reaction between GSH and methylparathion (dimethyl p-nitrophenyl phosphorothionate), 1-chloro-2,4-dinitrobenzene and S-crotonyl-N-acetylcysteamine were separated by (NH(4))(2)SO(4) precipitation from homogenates of sheep, rat and mouse livers and from homogenates of cockroaches, houseflies and grass grubs. 2. Electrofocusing of the preparations from each of these species separated a number of zones, each of which catalysed the reaction of GSH with all three substrates. 3. Ion-exchange chromatography on CM-cellulose also separated a number of fractions in which activity towards the three substrates coincided. 4. In both separation methods patterns of the activities were consistent with the presence in all species of several GSH transferases each having a degree of cross specificity towards the three substrates.

Role of hepatic anion-binding protein in bromsulphthalein conjugation

Using gel filtration, the binding of both glutathione and Bromsulphthalein (BSP) to a liver-soluble protein was found to be identical. BSP-conjugating activity (glutathione S-aryltransferase) was present only in the fractions corresponding to the two protein-bound markers. Using a highly sensitive assay, with 3,4-dichloronitrobenzene, the pattern of glutathione S-aryltransferase activity was found to coincide with Y protein. This evidence suggests that Y protein, or ligandin, has a dual role in hepatic transport: a specific enzymic function in the conjugation of certain anions with glutathione in addition to a transport function in the intracellular binding of organic anions.