SH-enzymes

Cadmium is extremely toxic and accumulates in humans mainly in the kidneys and liver prolonged intake, even of very small amounts, leads to dysfunction of the kidneys. It acts by binding to the —SH group of cysteine residues in proteins and so inhibits SH enzymes. It can also inhibit the action of zinc enzymes by displacing the zinc. 

Lewis SE. 1948. Inhibition of SH enzymes by methyl bromide. Nature 161 692- 693. 

Phenolic compounds naturally occurring in plants have induced many physiological responses that duplicate those reported for ozone and/or peroxyacetylnitrate (PAN). Chlorogenic acid is a competitive inhibitor of lAA-oxidase (35) and plant growth is adversely affected by increased concentrations of auxins (36). Concentrations of chlorogenic acid are increased in tobacco tissue exposed to ozone ( ) Phenols inhibit ATP synthesis (37), oxidative phosphorylation ( ) and SH enzyme activity (27) they increase respiration (38), reduce CO2 fixation (22), modify both membrane permeability (40) and oxidation rate of reduced NADH... 

Killer enzymes" or "anti-enzymes enzymes", enzymes that inactivate other enzymes. Of course proteases come immediately to mind. Also sulfhydrvl oxidase for -SH enzymes. Other specific enzymes aic possible. 

Both are probably SH enzymes, being inhibited by I2, Ag", p-chloro-mercuribenzoate and N-chlorosuccinimide, although not by iodoacetate or N-ethylmaleimide (85). Mercaptoethanol is commonly used in purification (83). The enzymes are also inhibited by Co ", Mn ", and Fe ", but chelating agents are without effect (85). Neutral salts such as NaCl also inhibit the mustard enzyme (87). 

The antineoplastic activity of naphthomycin A (56) and its mode of action were reported. The antibiotic caused neither metaphase arrest nor prevented tubulin polymerization, and it was suggested that the mechanism of cytotoxicity of 56 was the inhibition of various SH enzymes, particularly those involved in nucleic acid biosynthesis [228,229]. 

We have as yet no information as to the importance of such a system in the general respiratory activity of plant tissues. Its importance as a means of maintaining GSH, and hence of activating and conserving the activity of the so-called SH enzymes (enzymes which are dependent on the maintenance of certain SH groups in their molecules), is self-evident but does not concern us here. The participation of ascorbic acid in a respiratory chain of reactions will be prevented in the presence of cyanide, for the last stage, the enzymatic oxidation of ascorbic acid, will be inhibited. The extent to which hydrogen is transferred in the system will depend inter alia on the concentration of GSH and dehydroascorbic acid and on the... 

Reaction with thiol-moieties (alkylation) (Non)-protein -SH DNA SH-enzymes activity Folpet, epoxides... 

Three types of cell constituents react with SH-reagents 1. low M, thiols, such as lipoic acid, coenzyme A, glutathione and cysteine 2. non-enzyme proteins, such as actomyosin, membrane proteins and structural proteins 3. enzymes. The toxic action of SH-reagents in vivo is due chiefly to their effect on SH-enzymes. 

Thiol enzyme. SH-enzyme an enzyme whose activity depends on the presence of a certain number of free tUol groups. T.e. are found among the hydrolases, oxidoreductases and transferases. Known T.e. are bromelain, papain, urease, various flavoenzymes, pyridine nucleotide enzymes, pyridoxal phosphate enzymes and thiolproteinases. T.e. are t ically inhibited by Sulfhydryl reagents (see). 

The molecular interaction between the activating enzyme and the amino acid is not known, but it probably varies with the type of activating enzyme. Some enzymes (. g., the alanine-activating enzyme) are unaffected by paramercuric benzoate, but others, like the tryptophan-activating enzyme, appear to be SH enzymes the activity of which depends on the presence of free SH groups in the molecule. Potassium ions are known to activate the tyrosine enzyme. 

Thymidylate synthetase is an SH enzyme, and free SH groups are required for activity. The reaction involves the formation of a complex between the methylene derivative of folic acid and the enzyme, which in the presence of dUMP yields dTMP and a dihydro-folic acid enzyme complex. The properties of this enzyme will be discussed in more detail in other sections of this book. 

It is not my intent to belittle the pioneer efforts. The difficulties of the earlier workers seen retrospectively were the same as will be apparent to readers who judge us by this qumnt but historic Symposium twenty years hence. They were the consequences of insufficient information. The earlier work on SH and growth suffered from a tendency to assign specificity where it did not exist. SH reagents were presumed to be specific for particular enzymes then known to be SH enzymes, while poisons such as iodoacetate were presumed to be specific for certain enzymes because it was not realized that they were SH agents. 

Figure 12.2 Hypothetical mechanism of the nitrilase-catalyzed transformation of nitriles into carboxylic acids or amides. (According to [3].) E-SH enzyme with a cysteine as catalytic nucleophile.

Van-Hoek AN, de-Jong MD, van-Os CH (1990) Effects of dimethylsulfoxide and mercurial sulfhydryl reagents on water and solute permeability of rat kidney brush border membranes. Biochim Biophys Acta 1030 203-210 Verity MA, Sarafian T (1991) Role of oxidative injury in the pathogenesis of methylmercury neurotoxicity. In Suzuki T, Imura N, Clarkson TW (eds) Advances in mercury toxicology. Plenum, New York, pp 209-222 Waku K, Nakazawa Y (1979) Toxic effects of several mercury compounds on SH-and non-SH enzymes. Toxicol Lett 4 49-55 Warfvinge K, Hua J, Berlin M (1992) Mercury distribution in the rat brain after mercury vapor exposure. Toxicol Appl Pharmacol 117 46-52 Wasteneys GO, Cardin M, Reuhl KR, Brown DL (1988) The effects of methylmercury on the cytoskeleton of murine embryonal carcinoma cells. Cell Biol Toxicol 4 41-60... 

The oxidation of cysteine to cystine is catalysed by cytochrome-c and cytochrome-oxidase, whilst the reduction of C3rstine is accomplished by a number of reducing agents HjS, glutathione, SH-enzymes, etc. 

A great many enzymes (32, 46,144-152) have been treated with iodoacetate or iodoacetamide. Depending on whether or not they were inactivated by this procedure, they have been classified as SH enzymes (151). Janssen (153) has recently reported that iodoacetamide inactivates the foot and mouth disease virus and that this inactive product served as a vaccine. 

Urocanase appears to be an —SH enzyme. It is inhibited by p-chloro-mercuribenzoate and this can be reversed in part by glutathione. The enzyme was also inhibited by several carbonyl reagents—semicarbazid< cyanide, and hydroxylamine. The Michaelis constant for urocanic acio was found to have the value of 1.5 X 10 Af. 

A -Pyrroline-5-carboxylate reductase catalyzes the reduction of this compound to proline. In the rat liver enzyme DPNH and, less effectively, TPNH serve as hydrogen donors 115,120,123). In the Neurospora enzyme TPNH is sixteen times as effective as DPNH 124). A -Pyrroline-S-car-boxylate reductase has been observed in various mammalian tissues 120, 123) and in a variety of microorganisms 116, 123-126). The enzyme has been purified about thirtyfold, from Neurospora 126) and about eightyfold in the writer s laboratory from animal tissue 126). The enzyme is active on a number of A -pyrroline derivatives. The optimum activity of the liver and Neurospora enzymes is at about pH 7.0. The reductase is an SH enzyme and it is strongly inhibited by the usual SH reagents. Estimation of the kinetic constants of the liver enzyme yielded X values of 2.0 X 11" M for pyrroline-5-carboi late and 2.5 X 10 M for DPNH. 

Acetylornithinase has been more thoroughly characterized by Vogel and Bonner (130). The enzyme was purified thirty- to fortyfold by standard enzyme purification procedures. The activity of the enzyme was stimulated by Co" and by glutathione. Other evidence was obtained that this is an SH enzyme. The optimum activity is at pH 7.0. The K of acetylornithine was estimated to be 2.8 X 10 M. 

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