Sulphydryl enzymes

Practically all of the mercuric compounds are teratogenic in animals [241,242], Mercuric chloride thus induced cataracts and deaths in rat embryos [243]. In the human, mercuric chloride has been related to abortion [244, 245], possibly through the inactivation of placental sulphydryl enzymes. 

Sulphydryl enzymes are also inactivated by oxidation to the -S-S- form but addition of a mild reducing agent may serve to reactivate the enzyme. Sulphydryl groups are irreversibly inactivated by iodoacetamide and N-ethylmaleimide. 

Poisoning by arsenic compounds is occasionally encountered in the laboratory. The toxic action of arsenic is due to its inhibition of sulphydryl enzymes. It can be detected in body fluids by the Reinsch test which is based on the reduction of arsenic to its elemental form by metallic copper in the presence of acid. The arsenic is deposited on the coppier as a dark film. 

If the sequence of events in the liver is often enigmatic and the mechanism of haemolysis remains unexplained it must be admitted that the distribution of lesions in the brain is mysterious indeed. It has been suggested [56] that the reason why the brain and particularly the basal ganglia are so severely affected is because of their dependence on carbohydrate oxidation via the tricarboxylic acid cycle for their energy supply, the cycle operating via a number of sulphydryl enzymes. 

Oxidation of enzyme sulphydryl groups by free radicals (Armstrong and Buchanan, 1978) and by the longer lived chloroamines (Weiss et al., 1983), contributes to enzymic dysfunction and to the decrease in serum sulphydryl levels, primarily mercaptoalbumin, found in coalworkers with rheumatoid inflammatory disease (Thomas and Evans, 1975). Formation of the S-nitroso adduct of plasma mercaptalbumin is thought to provide a metastable reservoir for intravascular nitric oxide release (Stamler etal., 1992). 

A major contribution of the free-radical scavenging activity in blood plasma is attributable to the macro-molecular proteins (Wayner et al., 1985) of which albumin is a primary component and trapping agertt (Holt et al., 1984). Serum sulphydryl levels, primarily albumin-related, are decreased in subjects with rheumatoid complicated coalworkers pneumoconiosis, indicative of exacerbated inflammatory R.OM production (Thomas and Evans, 1975). Experimental asbestos inhalation in rats leads to an adaptive but evidendy insufficient response by an increase in endogenous antioxidant enzymes (Janssen etal., 1990). Protection of the vascular endothelium against iron-mediated ROM generation and injury is afforded by the iron sequestiant protein ferritin (Balia et al., 1992). 

The mercuric ion, Hg2 +, which is obtained after oxidation in the red blood cells and other tissues, is able to form many stable complexes with biologically important molecules or moieties such as sulphydryl groups. The affinity of mercury for sulphydryl groups is a major factor in the understanding of the biochemical properties of mercuric compounds, resulting in interference with membrane structure and function and with enzyme activity. 

Enzymes are important targets for mercury [71], and sulphydryl-group-containing enzyme being more sensitive to mercuric compounds than a non sulphydryl-group-containing enzyme [72], Enzymes reported to be inhibited include phosphatases [73, 74], dehydrogenases [75,76] and hexokinases [71]. 

Mercury can influence ion, water, and nonelectrolyte transport in different cells [ 14, 77]. The cell membrane is believed to be the first point of attack by heavy metals however, intracellular enzymes and metabolic processes may also be inhibited [70, 78, 79]. The attachment of heavy metals to ligands in or on the plasma membrane may result in changes in passive permeability or selective blockage of specific transport processes. Many membrane transport systems are known to be sensitive to sulphydryl-group modification [ 14, 80, 81]. 

Mercuric chloride has also been shown to inhibit the enzymatic activity of soluble protein kinase A from mice brain, apparently by binding to sulphydryl groups of the enzyme [111]. The inhibition was of a non-competitive type with respect to HI histone. 

Mercury and nickel salts form many stable complexes with biologically important molecules such as those containing sulphydryl groups Chapter 5 stresses the importance and the dangers of these being formed in the skin from topical contact. The last 10 years have been a highly fertile and productive period in the discovery of antibacterial quinolones (reviewed in Chapter 6) which inhibit target enzymes at the molecular level. 

The aminophosphohpid translocase is an ATPase Il-type enzyme that requires and is activated by phosphatidylserine and to a lesser extent by phosphatidylethanolamine and is sensitive to the sulphydryl group... 

There are a number of metals that are irreversible inhibitors of enzymes. Those that are found in increasing quantities in the environment and hence are causing concern include lead, mercury, cadmium and copper. Enzymes that possess the amino acid cysteine in their active site are most vulnerable, since the sulphydryl group (-SH) in this site readily reacts with the metal ions (Appendix 3.7). 

Deterioration (lipase (commercially, probably the most significant enzyme in milk), proteinase, acid phosphatase and xanthine oxidase) or preservation (sulphydryl oxidase, superoxide dismutase) of milk quality. 

Sulphydryl -(SH) groups appear to be essential for reactivation perhaps this is why phosphatase becomes reactivated in UHT milk but not in HTST milk. The role of -SH groups, supplied by denatured whey proteins, is considered to be chelation of heavy metals, which would otherwise bind to -SH groups of the enzyme (also activated on denaturation), thus preventing renaturation. The role of Mg2 + or Zn2 + is seen as causing a conformational change in the denatured enzyme, necessary for renaturation. 

Milk contains an enzyme, sulphydryl oxidase (SO), capable of oxidizing sulphydryl groups of cysteine, glutathione and proteins to the corresponding disulphide (reviewed by Farkye, 1992). The enzyme is an aerobic oxidase which catalyses the following reaction ... 

It undergoes marked self-association and can be purified readily by chromatography on porous glass. The enzyme has a molecular weight of about 89 kDa, a pH optimum of 6.8-7.0, and a temperature optimum of 35°C. Its amino acid composition, its requirement for iron but not for molybdenum and FAD, and the catalytic properties of the enzyme, indicate that sulphydryl oxidase is a distinct enzyme from xanthine oxidase and thiol oxidase (EC 1.8.3.2). 

Since the principal constituents of milk are proteins, lipids and lactose, proteinases, lipases and / -galactosidase (lactase) are the principal exogenous enzymes used in dairy technology. Apart from these, there are, at present, only minor applications for glucose oxidase, catalase, superoxide dismutase and lysozyme. Lactoperoxidase, xanthine oxidase and sulphydryl oxidase might also be included, although at present the indigenous form of these enzymes is exploited. 

Since sulphur compounds are important in the off-flavour of UHT milk, attempts to improve its flavour have focused on reducing the concentration of these, e.g. by adding thiosulphonates, thiosulphates or cystine (which react with mercaptans) or sulphydryl oxidase, an indigenous milk enzyme (which oxidizes sulphydryls to disulphides Chapter 8). 

They act on the kidney by depressing the mechanisms that govern the active reabsorption of sodium and chloride ions. They are rapidly excreted by the kidney but their use is hazardous because their action is believed to be due to inorganic mercury ions released by rupture of the carbon-to-mercury bond, probably followed by the firm attachment of the mercury ion to a sulphydryl group of a renal enzyme. The administration of dimercaprol (SO), a strong chelating agent for mercury, removes mercury from the kidney and terminates the diuretic action. It is of interest that Paracelsus used calomel (mercurous chloride) as a diuretic. 

O Neill P (1983) Pulse radiolytic study of the interaction of thiols and ascorbate with OH-adducts of dGMP and dG. Implications for DNA repair processes. Radiat Res 96 198-210 O Neill P (1984) Hydroxyl radical damage potential repair by sulphydryls, ascorbate and other antioxidants. Life Chem Rep Suppl Ser 2. In Rotilio G, Bannister JV (eds) Oxidative damage and related enzymes. Life Chem Rep 1 337-341... 

J. C. CaygilL Sulphydryl plant proteases. Enzyme Microb. Technol 7 233 (1979). 

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