688 5. OXIDANTS 



when more enzymes are susceptible to sequential analysis. Another interest- 

 ing approach to elucidating enzyme binding groups with iodine is illustrat- 

 ed by the study of the old yellow enzyme by Theorell (1956). Flavin is 

 bound to the apoenzyme through its imino group and fluorescence is quen- 

 ched; Weber had suggested that a tyrosine hydroxyl group might bind this 

 imino group. This was examined by reaction of the apoenzyme with iodine; 

 since no SH groups are present, this is relatively easy. It was found that 

 very low concentrations of iodine decrease the coupling rate of FMN to the 

 apoenzyme, and 90% of the iodine which disappears is recovered as diiodo- 

 tyrosine. 



Some results on the variation of inhibition with pH appear to point to 

 the importance of tyrosine iodination. The iodination of pepsin is very slow 

 below pH 4.5 and rises suddenly as the pH is increased to become maximal 

 around pH 5.5 (Herriott, 1937). This is essentially the same pH dependence 

 as found for glycyltyrosine. The inhibition of /3-fructofuranosidase by iodine 

 is minimal at pH 5.14 and much faster at pH's above 6 (Myrback, 1926), 

 which might support the importance of tyrosine iodination in the inhibition. 

 Although one might expect the effect of iodine on cathepsins to be mainly 

 through reaction with SH groups, Maver and Thompson (1946) found 

 greater inhibition by 0.25 mM iodine at pH 7 (71%) than at pH 3.5 (20%). 

 Results with various SH reagents do not favor the importance of SH 

 groups. This may well be a case where there is a mixed mechanism for the 

 inhibition. 



An interesting situation occurs in the reaction of the exopenicillinase of 

 B. cereus with iodine (Citri and Garber, 1960, 1961). This enzyme can exist 

 in two antigenically different states — a and y — and these differ in re- 

 sponse to iodine, although the enzyme activity is the same for both. a-Pen- 

 icillinase is quite resistant to iodine whereas ^-penicillinase is inhibited 

 by 0.5-1 mM iodine. The enzyme must be flexible since in the presence of 

 the competitive inhibitor, 6-(2,6-dimethoxybenzamido)penicillanic acid, it 

 becomes sensitive to iodine. Pretreatment with this inhibitor, followed by 

 its removal, does not alter hydrolysis of benzylpenicillin, so that any struc- 

 tural change that occurs is not permanent. It is very difficult to understand 

 how a rather large competitive inhibitor, which must cover the active cen- 

 ter, could allow reaction of any group at the active center with iodine, un- 

 less the reacted group is just vicinal to the active center and either SH oxi- 

 dation or iodination alters the structure. 



Effects on Cellular Metabolism and Function 



The uncoupling of oxidative phosphorylation by iodine has been claimed 

 to relate to the effects of thyroxine. Klemperer (1955) reported that al- 

 though iodide exhibits no uncoupling activity, iodine is quite effective in 

 rat liver mitochondria with /5-hydroxybutyrate as substrate (see accom- 



