Sulphate reductase

Reducing equivalents from a ven thioredoxin can be donated to a variety of reductase enzymes. They are not specific for the nucleotide reductase or for enzymes from the same organism. Reduced yeast thioredoxin will serve as reductant for methionine sulphoxide reductase, sulphate reductase and the E. coli nucleoside diphosphate reductase. Heat-stable protein cofactors are known to be involved in each of these systems. 

The sulphate reductase factor which has already been mentioned was the first of these polypeptide dithiol-disulphide cofactors to be recognized . In this case the reduction of PAPS to PAP and sulphite was shown possible with a dithiol reductant such as dihydrolipoate or with NADPH and two protein components. One of the protein factors was not inactivated by heating. Incubation of the two protein fractions with NADPH generated... 

The mechanisms involved are not fully worked out. It appears that all the intermediates remain bound to protein carriers. Evidence has been obtained that the initial reduction step sulphate reductase) involves two reactions, a low molecular weight hydrogen transporting protein, and reduced FAD as the electron donor. The details of the further reduction steps are even more obscure. It has been suggested that the whole 6-electron oxidoreduction is catalysed by a single eniyme [sulphite reductase) and that reduced NADP is the electron donor. 

Dihydrofolate reductase (from Mycobacterium phlei). Purified by ammonium sulphate pptn, then fractionated on Sephadex G-75 column, applied to a Blue Sepharose column and eluted with 1 mM dihydrofolate. [Al Rubeai and Dole BJ 235 301 7956]. 

Dihydropteridine reductase (from sheep liver) M 52,000 [EC 1.6.99.7]. Purified by fractionation with ammonium sulphate, dialysed versus tris buffer, adsorbed and eluted from hydroxylapatite gel. Then run through a DEAE-cellulose column and also subjected to Sephadex G-lOO filtration. [Craine et al. JBC 247 6082 7972]. 

Nitrate reductase, 1.9.6.1 Mo-pterin, Fe—S Nitrate PGE, neomycin sulphate [252]... 

When the Fe concentration in crotonate medium was growth limiting, cells with relatively high hydrogenase activity, and veiy low enoate reductase activity were obtained in the stationary phase. At Fe concentrations above 3 10 M enoate reductase increased, and hydrogenase activity reached its minimum. The ratio of activities changes by a factor of about 200. In a similar way the dependence of entyme activities on the concentration of sulphate was studied (23). 

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

GC-MS analysis is used to confirm the identities of ions in the LSI-MS urine spectrum and show that the excretion of abnormal cholanoids is >20 times normal. In the case of 5 ff-reductase deficiency GC-MS analysis should show that S-oxo-A" bile acids account for >70% of the total urinary bile acid excretion. In the case of sterol 27-hydroxylase deficiency (CTX), GC-MS analysis should indicate that the major cholestane pentols in the urine are 3,7,12,22,25 and 3,7,12,23,25-pentols. (One patient has been described who had familial cholestatic liver disease associated with greatly increased urinary excretion of 5jff-cholestane-3a,7a,12a,24 S,25-pentol [see previous table]). Liquid secondary ion-tandem mass spectrometry (LSI-MS/ MS) is an alternative method to GC-MS and can rapidly confirm the identity of a number of diagnostic ions that are found in the LSI-MS spectrum of urine. These include sulphated and taurine-conjugated abnormal metabolites such as those observed in 3 ff-HSDH deficiency (32.1), 5)9-reductase deficiency (32.2), oxysterol 7a-hydroxylase deficiency (32.4) and peroxisomal disorders [13]. 

Nitrite has an inhibitory effect on SRB, mainly because (a) nitrite is toxic to SRB, and with their nitrite reductase, the bacteria will produce a detoxifying reaction. The end result is that while the bacteria are still aUve, no growth happens and their sulphate reduction activity will be inhibited, (b) Nitrite can directly affect the enzyme required for reducing sulphite to sulphide see [63]. 

Selenium is readily absorbed, especially in the duodenum but also in the caecum and colon. Seleno-amino acids are almost completely absorbed selenomethionine via the gut methionine transporter and selenocysteine probably via the cysteine transporter. Both selenite and selenate are >50% absorbed, selenite more readily so than selenate, and for these forms there is competition with sulphate transport. Selenite is more efficiently retained then selenate because part of the latter is rapidly excreted into the urine. Vitamins A, E, and C can modulate selenium absorption, and there is a complex relationship between selenium and vitamin E that has not been entirely elucidated for man. A combined deficiency of both nutrients can produce increases in oxidative damage markers (malondialdehyde, Ei isoprostanes, and breath hydrocarbons) and in pathological changes that are not seen with either deficiency alone. Inorganic Se is reduced to selenide by glutathione plus glutathione reductase and is then carried in the blood plasma, bound mainly to protein in the very low-density lipoprotein fraction. Selenomethionine is partly carried in the albumin fraction. 

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