Mercury, enzyme inhibition

H. Mohammadi, A. Amine, S. Cosnier and C. Mousty, Mercury-enzyme inhibition assays with an amperometric sucrose biosensor based on a trienzymatic-clay matrix, Anal. Chim. Acta, 543 (2005) 143-149. 

S. Pirvutoiu, I. Surugiu, E.S. Dey, A. Ciucu, V. Magearu and B. Daniels-son, Flow injection analysis of mercury (II) based on enzyme inhibition and thermometric detection, Analyst, 126 (2001) 1612-1616. 

H. Mohammadi, M. El Razi, A. Amine, A.M.O. Brett and C.M.A. Brett, Determination of mercury (II) by invertase enzyme inhibition coupled with batch injection analysis, Analyst, 27 (2002) 1088-1093. 

Determination of methyl mercury using biphasic system (aqueous solution/ solvent) in the presence of fish matrix. The method is based on enzyme inhibition combined with glucose oxidase biosensor... 

The principle of combination of electrochemical glucose oxidase biosensor with the clean-up method for direct extraction and determination of methyl mercury has been successfully demonstrated. The extraction of methyl mercury from the organic solvent has been based on invertase enzyme inhibition. The combination of very low concentration of invertase enzyme and 10 min of incubation time allows the detection of methyl mercury at 5 ppb level. Our method permits the detection of this inhibitor below the legal limit given by the European Union with good recoveries when fish samples were measured. 

Nervous System. The nervous system is also a common target of toxic metals particularly, organic metal compounds (see Chapter 16). For example, methylmercury, because it is lipid soluble, readily crosses the blood-brain barrier and enters the nervous system. By contrast, inorganic mercury compounds, which are more water soluble, are less likely to enter the nervous system and are primarily nephrotoxicants. Likewise organic lead compounds are mainly neurotoxicants, whereas the first site of inorganic lead is enzyme inhibition (e.g., enzymes involved in heme synthesis). 

Pesticidal Mercury Compounds. Owens (98) established in 1953 that mercury produced in vitro inhibition of amino-dependent, sulfhydryl-dependent, iron-dependent, and copper-dependent enzymes. It is well known that mercury in high concentrations acts as a protein denaturant. For example, Sohler et al. (109) have shown that mercury compounds inhibit catalase activity at high concentrations. The inhibition of enzymes... 

Inhibition. Since papain, ficin, and bromelain are all enzymes whose activity depends on a free SH group, it is to be expected that all thiol reagents act as inhibitors. Thus, a-halogen acids or amides and N-ethyl-maleimide irreversibly inhibit the thiol proteases. Heavy metal ions and organic mercurial salts inhibit in a fashion that can be reversed by low molecular weight thiols, particularly in the presence of EDTA which... 

It is generally accepted that the basis of the biological activity of mercury compounds is their reaction with the thiol groups, but the biological action is rather more complicated. Frank (1955) showed that mercury compounds can influence the effect of enzymes which do not contain thiol groups. Mercury also reacts with the phosphoryl groups of the cell membranes (Bassow et a/., 1961) and with the amino and carboxyl groups of the enzymes (Lipscomb et al., 1968). Webb (1966) lists more than 40 enzymes inhibited by mercury compounds. 

The mechanism of thallium toxicity at the molecular level in animals and humans seems to be analogous to the enzyme-inhibiting effects of lead and mercury, and an oxidative stress, resulting in a general histotox-icity (Table 22.2) (Repetto et al. 1998, Leung and Ooi 2000, Appenroth and Winnefeld... 

Bertocchi R, Ciranni E., Compagnone D., Mageauru V., Palleschi G., Pirvutoiu S., and Valvo L., Elow injection analysis of mercury (II) in pharmaceuticals based on enzyme inhibition and biosensor detection, J. Pharm. Biom. Anal., 20, 263-269, 1999. 

I. Mechanism of toxicity. Mercury reacts with sulfhydryl (SH) groups, resulting in enzyme inhibition and pathologic alteration of cellular membranes. 

The number of proteins capable of metal interaction is enormous but actions of gold in arthritis (Chapter 11), and mercury in diuretics (Chapter 12.1), are two examples which may be traced to enzyme inhibition. The antibacterial effect of mercury is also probably due to enzyme inhibition (Chapter 9.1.1), contrasting with the inhibition of DNA synthesis by silver (Chapter 9.1.2). 

Mercuric chloride, other mercury-containing antibacterials and silver will inhibit enzymes in the membrane, and for that matter in the cytoplasm, which contain thiol, -SH, groups. A similar achon is shown by 2-bromo-2-nitropropan-l,3-diol (bronopol) and iso-thiazolones. Under appropriate condihons the toxic action on cell thiol groups may be reversed by addition of an extrinsic thiol compound, for example cysteine or thioglycollic aeid (see also Chapters 12 and 23). 

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]. 

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. 

That myosin, a structural protein, also had enzyme activity as an ATPase, had been shown by Engelhardt and Ljubimova (1939-1941). ATP was now found to dissociate actomyosin producing a marked fall in viscosity the ATP was split to ADP and Pj. Contrasting properties of ATP in muscle systems were also observed. The rigor seen at postmortem occurred as ATP levels fell. The ATPase activity of myosin could be inhibited by mercurials (which block SH groups on cysteine) with ATPase blocked, ATP caused muscle fibers to relax (Weber and Portzehl, 1952). 

The fact that ATP and CTP bind to the same site follows from the observation that adding ATP to the inhibited enzyme by CTP reduces or reverses the inhibition, presumably because ATP competes with CTP for the same site. The fact that CTP binds to an allosteric site (i.e., it is not a competitive inhibitor) follows from the so-called desensitization effect. Addition of mercurials [e.g., p-mercuribenzoate (PMB)] reduces or eliminates the inhibition by CTP. However, it has no effect on the enzymatic activity of ATCase, presumably because the mercurials affect the regulatory subunits but not the catalytic site. As for the mechanism of cooperativity (both positive and negative), it is known that CTP does induce changes in the quaternary structure of the enzyme. 

---