OXIDANTS 657 



(Hird and Yates, 1961). o-Iodosobenzoate usually oxidizes only to the di- 

 sulfide stage at pH 7, but if too high a concentration is used, or the pH is 

 much below 7, sulfinate or sulfonate groups are produced (Hellerman et at., 

 1941). In addition, methionine residues may be oxidized to the sulfoxide 

 stage. Sizer (1942 a,b, 1945) studied the effects on enzymes of many oxida- 

 tion-reduction systems over a wide range of Eq and found that as the Eq 

 is increased from around — 0.5 there is little effect on activity until a crit- 

 ical value is reached, which is +0.6 for /?-fructofuranosidase, +0.35 for 

 intestinal phosphatase, and +0.58 for chymotrypsin, inactivation increas- 

 ing rapidly above these values. Of course, it is not entirely a matter of the 

 Eq , the nature of the oxidant being also very important. The enzymes 

 used are not those containing SH groups most easily oxidized, but it does 

 indicate that rather strong oxidants must be used with many enzymes. 



Oxidants can inhibit enzymes by mechanisms other than oxidation of SH 

 groups. They may (1) oxidize other enzyme groups, (2) be chemically in- 

 corporated into the enzyme (e.g., the iodination of tyrosine residues by 

 iodine), or (3) inhibit reversibly by any of the mechanisms observed with 

 nonoxidizing inhibitors. Other enzyme groups susceptible to oxidation are 

 the hydroxy! groups of tyrosine and serine, the hydrocarbon chain of leu- 

 cine, the indole ring of histidine, and perhaps the amino, guanidine, and 

 peptide groups. A few examples will be mentioned and others will be dis- 

 cussed in the sections on the individual oxidants. Lieben and Bauminger 

 (1933 a) showed that several amino acids are attacked by ^permanganate. 

 During the oxidation of casein, the arginine content falls, urea and dixan- 

 thylurea appearing. Haas et al. (1951) emphasized the importance of tyro- 

 sine and tryptophan in the actions of permanganate on proteins. Although 

 phenylalanine is quite refractory, tyrosine and tryptophan are oxidized, as 

 shown by changes in the ultraviolet spectra. The spectra of insulin and pep- 

 sin treated with permanganate (0.1 mM at pH 2) change in a manner sim- 

 ilar to that of the free amino acids, so it is likely that oxidation of these 

 amino acids occurs when they are part of the protein structure. Oxidation 

 of several proteins by periodate releases formaldehyde, which probably arises 

 from hydroxylysine (Desnuelle and Antonin, 1946). One mole of ovalbumin 

 reduces 30 moles of periodate to iodate, the protein losing all of its cysteine 

 and cystine, one third of its tryptophan, and a small fraction of its tyrosine 

 (Desnuelle et al., 1947). Oxidation of seralbumin by periodate results in 

 destruction of certain amino acids, producing changes in spectral and elec- 

 trophoretic properties (Goebel and Perlmann, 1949). Periodate releases acet- 

 aldehyde from chymotrypsin, arising from terminal threonine, although this 

 is not responsible for the inhibition of the enzyme inasmuch as it occurs 

 before inactivation starts, and in this case no ultraviolet spectral changes 

 are observed (Jansen et al., 1950, 1951). Nitrous acid not only oxidizes cer- 

 tain enzyme groups, such as SH, but attacks free tyrosine and amino groups 



