Levels of glutathione, glutathione reductase and glutathione S-transferase activities in rat lung and liver.

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C, synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress.A description of the mechanisms of transcriptional and posttranscriptional regulat...

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance.

In this review, we have examined the biochemical and toxic responses produced by halogenated aromatic hydrocarbons and have tried to develop a model for their mechanism of action. These compounds bind to a cellular receptor and evoke a sustained pleiotropic response. In many tissues this response consists of the expression of a battery of enzymes which are, for the most part, involved in drug metabolism, but in other tissues, those which develop toxicity, an additional set of genes is expressed which effects cellular involution, division, and/or differentiation. The toxicity of these compounds appears to be due to the sustained expression of a normal cellular regulatory system, of which we were previously unaware. In future investigations it is hoped that we will learn the nature and physiologic role of this regulatory system. Only then can we hope to understand the mechanism of toxicity of these compounds.

2,3,7,8-Tetrachlorodibenzo-p-Dioxin and Related Halogenated Aromatic Hydrocarbons: Examination of the Mechanism of Toxicity

Publisher Summary This chapter provides the spectrophotometric, titrimetric, nitrite, and cyanide assay for the differentiation of glutathione S-transferases. Spectrophotometric assays depend upon a direct change in the absorbance of the substrate when it is conjugated with glutathione (GSH). Because each of the reactions is catalyzed at a finite rate in the absence of enzyme, care is needed to reduce nonenzymatic catalysis by minimizing substrate concentrations and by decreasing pH wherever necessary. Titrimetric assay is based on the principle that the conjugation of alkyl halides with GSH can be measured titrimetrically. Although acid production accompanies many of the transferase catalyzed reactions in which thioethers are formed, titrimetry is only used when more convenient assays are not available. Nitrite assay is based on the principle that nitrite is released when GSH reacts with nitroalkanes or with organic nitrate esters. The nitrite can be assayed as the limiting factor in a diazotization reaction with sulfanilamide that produces a readily quantitatable pink dye. Cyanide assay is based on the fact that when glutathione transferases catalyze the attack of the glutathione thiolate ion on the electrophilic sulfur atom of several organic thiocyanates, it results in the formation of an asymmetric glutathionyl disulfide and cyanide. Cyanide can be readily quantitated by a calorimetric method.

Assays for differentiation of glutathione S-transferases.

Glutathione metabolism and its selective modification.

Induction, stabilization and turnover of endoplasmic reticulum proteins*

Attempts to Induce Hepatic Uridine Diphosphate Glucuronyl Transferase in Genetically Deficient Gunn Rats by Grafting of Normal Liver Tissue

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Attempts to induce hepatic uridine diphosphate glucuronyl transferase in genetically deficient Gunn rats by grafting of normal liver tissue.

The mechanisms through which bilirubin is taken up and conjugated by the liver have become better understood in recent years, partly because specialized protein receptors have been identified, partly because mutant animals having liver impairments similar to some found in man are available. More needs to be learned, however, concerning the mechanisms of excretion before bilirubin metabolism is fully clarified.

Bilirubin Metabolism: Research Aspects

Chapter 2 – Dismutation of Bilirubin Monoglucuronide

Gamma-glutamyl transpeptidase (γGTP), an intestinal mucosal enzyme, hydrolyzes γ glutamic carboxyl amide bonds. Subfractions of gluten were obtained by a modification of the hydrolytic scheme of Frazer. A high glutamic acid concentration characterizes gluten (38%) with enrichment of this amino acid in the residue following hydrolysis (68–72%). A γ-carboxyl peptide configuration appears in 2 of the subfractions. An in vitro assay syste utilizing the concept of enzyme inhibition showed the specific hydrolytic action of γGTP to be the γ-glutamic acid site in gluten and its subfractions. When γ-glutamyl beta naphtylamine served as substrate, with guinea pig intestinal mucosal scrapings supplying the enzyme, several of the subfractions acted as potential inhibitor compounds. Intestinal γGTP was determined in jejunal biopsies from 10 adult human subjects, 4 controls and 6 patients with small intestinal pathology including 4 with celiacsprue. Enzyme activity was found to be approximately half that of the normal in 2 sprue patients in clinical remission (normal fecal fat balance, normal morphological appearance of the jejunal biopsy and normal intestinal maltase activity). In the other 2 sprue patients the subjects were not in remission and the γGTP levels were even lower. These preliminary data suggest γGTP may be required to hydrolyze the γ glutamyl moieties in gluten and that a deficiency of this enzyme may result in the clinical entity of celiac-sprue. Whether there is a genetic-molecular basis for celiac-sprue, an acquired deficiency of enzyme, or both, will be answered when additional data become available.

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Irwin M. Arias

Papers

Induction, stabilization and turnover of endoplasmic reticulum proteins*

Attempts to induce hepatic uridine diphosphate glucuronyl transferase in genetically deficient Gunn rats by grafting of normal liver tissue.

Bilirubin Metabolism: Research Aspects

Chapter 2 – Dismutation of Bilirubin Monoglucuronide

Celiac-sprue Syndrome: Etiologic Considerations