Nitroprusside reaction with thiols

The ability of coordinated NO to react with thiols has led to the suggestion of an alternative mechanism for activating guanylate cyclase. This involves nitroprusside oxidation of protein sulfhydryls to cross-link the protein with a disulfide bridge. For example, papain, which has an essential cysteine (cys-25) and glyceradehyde-3-phosphate dehydrogenase (cys-149) are both inhibited by nitroprusside with formation of [Fe(CN)5(NO)] and [Fe(CN)4NO] [132]. The suggested anaerobic reaction is ... 

Other Properties and Problems. The low redox potential of dithiothreitol interferes with its use in the presence of certain reagents. While it gives full color yield in reactions with aromatic disulfides such as 5,5 -dithiobis(2-nitrobenzoate) or 2,2 - or 4,4 -dipyridyl disulfide, gives only 4% the expected color in the nitroprusside assay for thiols, presumably because it reduces the iron. It reduces Ci or Co , complexes of nucleotides... 

It should be stressed that in order to use the delayed nitroprusside reaction for the detection of acetyl CoA it was necessary first to remove the large amounts of the free mercaptans, mostly glutathione, from the yeast Kochsaft which otherwise would have strongly interfered with the color reaction. We achieved this by titration of the Kochsaft with iodine, thereby oxidizing thiols to disulfides which do not any longer react with nitroprusside at alkaline pH. The identity of the sulfhydryl component of our active preparations with coenzyme A was finally proved in various ways which should not be discussed here in detail. 

Since thiolates are far more reactive than thiols (pK 8-9.5) in nucleophilic attack on coordinated NO, these rates are pH dependent [53]. The reaction of alkaline solutions of mercaptans with nitroprusside to yield reddish-purple solutions has long been used as a test for cysteine, glutathione or other thiol-containing compounds. However, if the solution is too basic, the nitroprusside/ hydroxide reaction becomes competitive [3]. 

Another EPR signal, at gL = 1.999 and g = 1.927, with resolved " N hyper-fine splitting, is observed for an intermediate Fe NO" complex during the reduction of nitroprusside by thiols [58]. Reaction of nitropmsside with a thiol such as cysteine leads to a one-electron reduction of the complex and formation of... 

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