The inhibition of 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) dehydrochlorinase and glutathione S-aryltransferase in grass-grub and housefly preparations

Inhibition of human glutathione S-transferases by basic triphenylmethane dyes

Human glutathione S-transferases (GSTs) of the Alpha-, Mu- and Pi- classes were expressed in E. coli and isolated by affinity chromatography. They were tested for their susceptibility to inhibition by basic triphenylmethane dyes. hGSTA 1-1 was inhibited by Malachite Green with a Ki value of the order of 10 microM. The inhibitory species appeared to be the dye-GSH adduct. This isoenzyme was not inhibited by either Crystal Violet or Ethyl Violet at concentrations up to 50 microM. hGSTM 2-2 was weakly inhibited by all three dyes tested with Ki values being in the range 40-80 microM. For all dyes the inhibition was best characterised as non-competitive. hGSTP 1-1 was not inhibited by Crystal Violet or by Ethyl Violet but was strongly inhibited by Malachite Green (Ki = 0.3 microM). The mode of inhibition appeared to be non-competitive but it seems probable that the mechanism is complex. There is at present no evidence to show clearly whether the dominant inhibitory species is the free dye or the adduct.

Inhibition of glutathione S-transferases from rat liver by basic triphenylmethane dyes

(1) Basic triphenylmethane dyes related to pararosanilin inhibit class alpha glutathione S-transferases (GSTs) from rat liver. The inhibitory potency of each dye correlates with its octanol-water partition coefficient. Values of Ki determined at pH 6.5 ranged from about 1 x 10(-7) M for Ethyl violet to 7 x 10(-5) M for Methyl green. GST 3-3, a class mu isoenzyme, was an order of magnitude less sensitive to inhibition by Ethyl violet. (2) All of the dyes tested were bleached to varying degrees by glutathione. The bleaching appears to result from the formation of an adduct between the dye and glutathione. At pH 6.5, adduct formation is significant only for Malachite green and Methyl green. There is kinetic evidence that for these dyes the adduct contributes significantly to the overall inhibition. It is probable that at physiological pH, all of the dyes would exist to a significant extent in the adduct form. (3) The dyes are excreted extensively in the bile, at least partly as the glutathione adduct. The free dye is regenerated on standing, it is assumed as a result of removal of glutathione by oxidation.

Glutathione transferases--structure and catalytic activity

The glutathione transferases are recognized as important catalysts in the biotransformation of xenobiotics, including drugs as well as environmental pollutants. Multiple forms exist, and numerous transferases from mammalian tissues, insects, and plants have been isolated and characterized. Enzymatic properties, reactions with antibodies, and structural characteristics have been used for classification of the glutathione transferases. The cytosolic mammalian enzymes could be grouped into three distinct classes--Alpha, Mu, and Pi; the microsomal glutathione transferase differs greatly from all the cytosolic enzymes. Members of each enzyme class have been identified in human, rat, and mouse tissues. Comparison of known primary structures of representatives of each class suggests a divergent evolution of the enzyme proteins from a common precursor. Products of oxidative metabolism such as organic hydroperoxides, epoxides, quinones, and activated alkenes are possible "natural" substrates for the glutathione transferases. Particularly noteworthy are 4-hydroxyalkenals, which are among the best substrates found. Homologous series of substrates give information about the properties of the corresponding binding site. The catalytic mechanism and the active-site topology have been probed also by use of chiral substrates. Steady-state kinetics have provided evidence for a "sequential" mechanism.

[The enzymatic degradation of DDT. 5. Direct transformation of DDD (dichlorodiphenyldichloroethane) into an aldehyde]

The chemical-preparative preparation of the aldehyde (DDHO) is possible by using of saponification. Its enzymatic formation in the organism of warm-blooded animals has not been described up to now. The identification of the aldehyde after application of DDD to rats was successful, inspite of the inhibition of the HCl-cleaving consecutive reactions, whereby the enzymatic formation of the aldehyde was indicated. A possible reaction mechanism is presented.

Inhibition of housefly oxidative detoxication by phthaleins, fluoresceins and related compounds

1. Phenolphthalein, halogenated fluoresceins, and other triphenylmethane and diphenylmethane derivatives inhibited biphenyl hydroxylation, aldrin epoxidation and several O-dealkylations in insect abdomen homogenates. Phenolphthalein and eosin (50 muM) were 2-3 times more effective than SKF 525-A and piperonyl butoxide (50 muM) as inhibitors of biphenyl hydroxylation in vitro. 2. The phthaleins, Aurin and Aluminon, inhibited both epoxidation and hydroxylation to similar extents, but fluoresceins, Rhodamine B, Malachite Green, and basic diphenylmethane derivatives preferentially inhibited hydroxylation. 3. Tetrabromophenolphthalein ethyl ester and bis-(N-dimethyl-4-aminophenyl-methane inhibited biphenyl hydroxylation in vivo. Bis-(N-dimethyl-4-aminophenyl) methane synergized the toxic effects of 1-naphthyl N-methylcarbamate in live houseflies.

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.

Reversible inhibition in bimolecular rapid equilibrium random order enzyme systems. The effect of substrate-substrate and inhibitor-substrate interactions

A model is presented that accounts for all types of reversible inhibition by a single inhibitor molecule in bimolecular rapid-equilibrium random-order enzyme systems. The characterization of inhibition mechanisms by graphical methods is examined, and a system of nomenclature is suggested.

The separation of multiple forms of housefly 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) dehydrochlorinase from glutathione S-aryltransferase by electrofocusing and electrophoresis

Electrophoresis in a sucrose gradient at pH values between 5 and 8 separated housefly DDT [1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] dehydrochlorinase into two major fractions. GSH S-aryltransferase under similar conditions migrated as a single peak of activity. Separation of housefly homogenates or partially purified enzyme preparations by electrofocusing in a natural pH gradient also showed the presence of multiple forms of DDT dehydrochlorinase.