IODINE 683 



stage would not be expected to be reversed. No reactivation of /3-galactosi- 

 dase (Knopfmacher and Salle, 1941 ) or a-amylase (Di Carlo and Redfern, 

 1947) is observed; however, in both cases there is some reason for believing 

 that SH groups are involved. Partial reactivation of /5-amylase (Weill and 

 Caldwell, 1945 b) and phosphoglyceraldehyde dehydrogenase (Rapkine, 

 1938) was taken to mean that at least SH group oxidation is responsible 

 for the inhibition. Essentially complete reactivation with thiols has been 

 found for urease (Hellerman, 1939), papain (Hellerman and Perkins, 1934), 

 cholinesterase (Nachmansohn and Lederer, 1939), and lactate dehydrogen- 

 ase (Nygaard, 1955) so that an SH mechanism seems assured for these. 

 The mechanism of the inhibition of /5-fructofuranosidase is still unknown, 

 although it is the first enzyme studied with iodine. The enzyme is inhibit- 

 ed fairly rapidly to the extent of 45 50%, but further inactivation proceeds 

 very slowly (Myrback, 1926). A "Jodsaccharase" was assumed, but the 

 iodine must not be fixed at the active center since there is no decrease in 

 the affinity for the substrate. There is no reactivation by reduction (Myr- 

 back, 1957 a) so there is little evidence for SH group oxidation. Sulfenyl 

 iodide groups may be involved. 



There has been little study of the disappearance of SH groups during in- 

 hibition by iodine. Cardiac lactate dehydrogenase SH groups are rapidly 

 oxidized by iodine, as determined with Ag+ and spectrophotometrically with 

 p-mercuribenzoate, and the inhibition develops in parallel fashion (Nygaard, 

 1956). The lactate dehydrogenase from rabbit muscle, on the other hand, 

 incorporates iodine at 0^ and pH 8 over many hours; when 1 atom of iodine 

 is incorporated per molecule of enzyme, the inhibition is 30%, and the in- 

 hibition increases until 21 atoms of iodine are incorporated. Both NAD and 

 oxalate protect the enzyme against iodination. Although the results with 

 iodoacetamide indicate an SH group at the active site, one cannot be cer- 

 tain if this is the initial point of attack for the inhibition (Dube et al., 1963). 

 It is likely in situations like this that both SH group oxidation and iodina- 

 tion of tyrosine occur. 



Pepsin is not an SH enzyme but is inhibited by iodine, and here it is 

 highly probable that tyrosine iodination occurs. The activity of pepsin de- 

 creases with the amount of iodine incorporated; it is inactive when 35-40 

 atoms of iodine are bound (Herriott, 1937). 3-Iodotyrosine has been isolat- 

 ed from inhibited pepsin (Herriott, 1947), confirming the importance of 

 tyrosine for the enzyme activity. Of the 6 tyrosyl groups in ribonuclease, 

 3 are unreactive, and the problem of where these are in the polypeptide 

 chain was studied with iodine (Cha and Scheraga, 1961 a,b). At pH 9.4 and 

 10° — conditions favorable for tyrosine iodination with minimal effects on 

 other groups — 3 tyrosyl residues are iodinated; the others can be iodinated 

 only very slowly. The iodinated tyrosyl residues were located in the amino 

 acid sequence. Such techniques will undoubtedly become more common 



