682 5. OXIDANTS 



fides. Intermolecular disulfide formation was ruled out for these proteins. 

 Ovalbumin has 4 SH groups and 2 S — S groups; the protein treated with 

 iodine has 2 SH groups and 2 S — S groups, and has incorporated 1 iodine 

 atom (Winzor and Creeth, 1962). Since 5 atoms of iodine are taken up, it 

 is not a simple oxidation to disulfide. It was suggested that the following 

 reactions occur: 



2 P(— SH)2 



HS-P— S— S-P-SH 



IS-P-S-S-P— SI 



+ 2 I2 + 3 HjO 

 IS-P— SO + bzS-P-SI 



where P represents that portion of the protein not reacting with iodine. 

 Further oxidation of the sulfenate group to sulfinate may occur to give a 

 homogeneous product. Therefore the formation of sulfenyl iodide groups 

 and oxidation of SH groups to sulfenate and sulfinate must be considered 

 as likely possibilities in enzymes treated with iodine. 



Inhibition of Enzymes 



Many enzymes have been found to be readily inhibited, often by low con- 

 centrations of iodine (Table 5-4). It is impossible to know in most cases 

 whether the inhibition is due to reaction with SH groups or to iodination 

 of tyrosine. The fact that most studies have been done at pH's around 

 neutrality implies that both SH and tyrosyl groups could be reacted, so 

 that the relative importance would depend on the accessibility of the groups 

 and their location with respect to the active center. Fixation of iodine into 

 an enzyme does not imply inhibition; an example is Aspergillus protease 

 (Dhar and Bose, 1962). The inhibition of certain enzymes by iodine is 

 probably related to oxidation of SH groups: papain, creatine kinase, urease, 

 aldolase, lactate dehydrogenase, succinate dehydrogenase, pyruvate decar- 

 boxylase, and adenosinetriphosphatase. Other enzymes, such as pepsin or 

 peroxidase, are inhibited through tyrosine iodination, and in some instances 

 a mixed mechanism is probable. 



One way of determining if SH group oxidation is responsible for enzyme 

 inhibition is to attempt reversal with thiols. Complete reversal certainly 

 implies such a mechanism, but negative results can be interpreted in various 

 ways. Even oxidation to disulfide groups is not necessarily reversed by 

 thiols if steric factors prevent reaction, and oxidation past the disulfide 



