Membrane thiol groups

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). 

Mention has been made of thiol groups in the cytoplasmic membrane as targets for certain antibacterial compounds. Thiol groups also occur in the cytoplasm and these groups will also serve as targets. 

Moroney, J.V., Wamcke, K., and McCarthy, R.F. (1982) The distance between thiol groups in the gamma subunit of coupling factor 1 influences the protein permeability of thylakoid membranes. J. Bioenerg. Biomembr. 14, 347. 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

Fung and colleagues examined the metabolic conversion of organic nitrates in sub-cellular fractions of bovine coronary artery smooth muscle cells [66, 67]. They found NO-generating capacity to be present in membrane fractions and, with the use of marker enzymes, identified plasma membrane as the primary location. The enzyme involved in bioconversion was not glutathione-S-transferase [68] and differed from those that catalyse activation of organic nitrites [69]. Partial purification [70] established that the molecular sizes of the native enzyme and subunits were approximately 200 kDa and 58 kDa respectively, and that enzymic activity depends on the presence of a free thiol group. 

This process is an early morphological change in cells often seen in isolated cells in vitro but also known to occur in vivo. The blebs, which appear before membrane permeability alters, are initially reversible. However, if the toxic insult is sufficiently severe and the cellular changes become irreversible, the blebs may rupture. If this occurs, vital cellular components may be lost and cell death follows. The occurrence of blebs may be due to damage to the cytoskeleton, which is attached to the plasma membrane as described above. The cause may be an increase in cytosolic Ca2+, interaction with cytoskeletal proteins, or modification of thiol groups (see below). 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

However, an inhibitory effect of high heat treatment on the oxidative deterioration of milk and milk products has been reported by many investigators who have attributed this effect to the activation of thiol groups (Josephson and Doan, 1939 Tamsma et al., 1962 Wilson and Herreid, 1969 Schwartz and Parks, 1974 Baldwin and Ackland, 1991 Saidi and Warthesen, 1995 Tong et al., 2000). The principal sources of thiols in milk are the fat globule membrane (McPherson and Kitchen, 1983) and the serum proteins, particularly (3-lactoglobulin (Larsson and Jenness, 1950 Schwartz and Parks, 1974). 

Interaction with thiol groups Mercurials (q. v.) Membrane-bound enzymes... 

Type II FDP aldolases are more stable than their type I counterparts. For example, the enzyme from E. coli has no thiol group in the active site and has a half-life of approximately 60 days in 0.3 mM Zn+2 at pH 7.0. The type I enzyme from rabbit muscle (RAMA), by contrast, has a half-life for the free enzyme of approximately 2 days in aqueous solution at pH 7.O.20 These half-lives can be lengthened by immobilization or enclosure in dialysis membranes. 

When analysing the thiol content of membrane proteins, underestimates may arise due to the inaccessibility of some of the SH groups within the hydrophobic portion of the membrane. The following method is used for accurate assessment of membrane protein thiol groups (Deuticke et al 1988)... 

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