Thiol enzymes

Although requirements such as those mentioned may be desirable or even essential for an active plant growth regulator, the mechanism of action of the substance in the cell is still unknown. The suggestion that certain of these active molecules may exercise their effects by combination with important thiol enzymes may be most fruitful. 

Garlic s proven mechanisms of action include (a) inhibition of platelet function, (b) increased levels of two antioxidant enzymes, catalase and glutathione peroxidase, and (c) inhibition of thiol enzymes such as coenzyme A and HMG coenzyme A reductase. Garlic s anti-hyperlipidemic effects are believed to be in part due to the HMG coenzyme A reductase inhibition since prescription medications for hyperlipidemia have that mechanism of action (statins). It is unknown whether garlic would have the same drug interactions, side effects, and need for precautions as the statins. 

Volume 348. Protein Sensors and Reactive Oxygen Species (Part B Thiol Enzymes and Proteins)... 

Analyses of enzyme reaction rates continued to support the formulations of Henri and Michaelis-Menten and the idea of an enzyme-substrate complex, although the kinetics would still be consistent with adsorption catalysis. Direct evidence for the participation of the enzyme in the catalyzed reaction came from a number of approaches. From the 1930s analysis of the mode of inhibition of thiol enzymes—especially glyceraldehyde-phosphate dehydrogenase—by iodoacetate and heavy metals established that cysteinyl groups within the enzyme were essential for its catalytic function. The mechanism by which the SH group participated in the reaction was finally shown when sufficient quantities of purified G-3-PDH became available (Chapter 4). 

Further search for inverse substrates other than /7-amidinophenyl esters has been carried out and it has been found that esters derived from p-amin omcthylphcnol and /7-guanidinophenol were also eligible as a substrate of trypsin and trypsin-like enzymes 75 86). We have also found that trimethylaminobutanoic acid p-nitrophenyl ester is an inverse substrate for butyrylcholinesterase 87-88(. Application of the inverse concept to thiol enzymes was also successful p-amidinophenyl esters were found to be substrates for clostripain 74), a thiol enzyme with trypsin-like specificity. Although the design of inverse-type substrates seems not always possible for a variety of hydrolytic enzymes, this new concept could provide potential means for certain enzymes to both fundamental study and application. 

R3MX are the most poisonous in the R4 MX (M = Sn, Pb) series this is also observed for the isostructural compounds of silicon . The toxic action of compounds of tin and lead is similar. Derivatives of R3MX inhibit oxidative phosphorylation, whereas R2MX2 binds thiol enzymes groups. Fungicidal activity of trialkylstannane R3MX derivatives is maximal when the number of carbon atoms is 9 in all three R substituents. On the whole, the toxicity of organic compounds of tin and lead decreases as the bulk of the substituents about the central metal atom increases. 

The reaction accounts for the high toxicity of lead and mercury to living organisms because they react with vital cellular thiol enzymes, thereby poisoning them. 

Pullar, J.M., Winterboum, C.C., and Vissers, M.C.M. (1999). Loss of GSH and thiol enzymes in endothelial cells exposed to sublethal concentrations of h3fpochlorous acid. Am J Physiol Heart Circ Physiol 277(4) H1505-12. 

Mackworth JR (1948). The inhibition of thiol enzymes by lachrymators. Biochem J, 42, 82-90. 

Bromelain (EC 3.4.22.4) a thiol enzyme from the stems and fruit of the pineapple plant. The stem enzyme is a basic glycoprotein (M, 33,000, pi 9.55) structurally and catalytically similar to papain. B. is activated by mercaptoethanol and other SH compounds, and it is irreversibly inhibited by reagents that block SH groups. It is an endopeptidase, and it is used in protein chemistry to hydrolyse polypeptide chains into large fragments. 

EndopepUdases (proteinoses) catalyse the hydrolysis of bonds within the peptide chain, forming variously sized cleavage peptides. They can be further subdivided into acidic, neutral and basic endopeptidases. Neutral and basic types can each be divided into Serine proteases (see) and thiol proteinases (see Thiol enzymes). Examples of animal endopeptidases are Pepsin (see). Rennet enzyme (see), Ttypsin (see), Elastase (see). Thrombin (see), Plasmin (see) and Renin (see). For examples of plant and bacterial endopeptidases, see Papain, Subtilisin, Bromelain. Endopeptidases have also been isolated from yeast and fungi. 

Alloxan, well known for its ability to specifically damage the insulin secreting fl-cells in the pancreatic islets, is an SH-reagent. It shows high specificity, being quite inactive towards many known thiol 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). 

Thiol group, sulfkydryl group, metcapto gro -SH, the functional group of thiols (mercaptans), i.e. the functional group of RSH, where R is the remainder of the molecule. T.g. may be structurally important as in Thiol enzymes (see), or functionally important as in Coenzyme A (see), Pantetheine-4 -phosphate (see), Lipoic acid (see), Thioredoxin (see), etc. The functional form of lipoic and thioredoxin is a dithiol. 

Glyceraldehyde phosphate dehydrogenase probably holds the distinction of being the classic thiol enzyme in the minds of most biochemists . The thiol is believed to be involved in the initial attachment of the aldehyde substrate as a thiohemiacetal. The em me-bound thiohemiacetal is then oxidized by NAD generating an enzyme-bound thioester. In more sophisticated proposals for this mechanism the nicotinamide cofactor interacts with the active centre thiol as a charge transfer type of complex. This facilitates the reaction of the thiol with the carbonyl of the substrate. The thiol addition and the electron transfer to nicotinamide occur... 

The thiol enzyme for which the most detailed mechanistic formulations have been proposed is papain . In this enzyme a cysteine thiol group appears to function in the same manner as the serine hydroxyl of other proteases and esterases. In the hydrolysis of proteins by this plant protease there is an intermediate formation of an acyl thiol, which is subsequently cleaved by water. 

Treatment of the enzyme with acyl phosphate in the complete absence of reduced cofactor has allowed the thiol enzyme derivative to be prepared and separated from its reaction mixture. This in turn has permitted considerable characterization of the enzyme thiol. No special cofactor is involved. The thiol of a cysteine residue from the main peptide chain of the enzyme provides the reactive centre. This enzyme demonstrates that the acyl transfer role of thioesters in biological systems is not restricted to phosphopantetheine and dihydrolipoate derivatives. The reactions of the... 

Rhodanese provides an example of a thiol enzyme of a somewhat different type. This enzyme, which is widely distributed throughout nature, catalyses the formation of thiocyanate from thiosulphate and cyanide. This reaction probably does not represent the true biological... 

Ueno, Y. and Matsumoto, H. (1975). Inactivation of some thiol-enzymes by trichothecene mycotoxins for Fusarium species. Chem. Pharm. Bulletin. 23, 2439-2442. 

The in vitro inactivation of various thiol enzymes has been well demonstrated for radiation (7), and for high oxygen tensions (1, 2, 3). For the latter the evidence available points to pyruvic oxidase as constituting that part of the brain respiratory system most sensitive to oxygen. The role of an in vivo inactivation of these enzyme systems has been questioned, and several investigators have failed to demonstrate it for X-irradiation and Stadie (1), for high oxygen tensions. 

Fontecave M, Gerez C. 2002. Tyrosyl radicals and ribonucleotide reductase, hnProtein sensors and reactive oxygen species, Pt B, thiol enzymes and proteins, pp. 21-30. Heidelberg Elsevier Science. 

B.M. Sjoberg and M. Sahlin, Protein Sensors Reactive Oxygen Species, Pt B, Thiol Enzymes Proteins, Methods. Enzymol, 2002,348,1. 

The several thiolases which have been described differ according to their chain-length specificity. The enzymes are all thiol enzymes (RSH) and the inter-... 

---