IODINE 681 



ciably altering tyrosine residues (Anson, 1941). Iodine reacts only with the 

 tyrosine residues of seralbumin, and denaturation accelerates the formation 

 of diiodotyrosine (Li, 1945). The rate of the reaction is, however, quite slow 

 in native seralbumin (half-reaction time around 100 min). Human seral- 

 bumin iodinated at low temperature takes up 36 atoms of iodine per mole 

 of albumin, but only 12 diiodotyrosyl groups are found (Hughes and Straes- 

 sle, 1950). The remainder was believed to be incorporated into histidyl re- 

 sidues. Some oxidation of cysteinyl residues also occurs and this presumably 

 is beyond the disulfide stage, since 2.2 moles of iodine are taken up per SH 

 group. In any particular case, the amount of disulfide formed will depend 

 to a large extent on steric factors, i.e., how readily the sulfhydryl radicals 

 can combine; the seralbumin molecule is fairly large and, not surprisingly, 

 disulfide groups are not found after oxidation. Although no degradative 

 changes in seralbumin are observed, protein structure is certainly modified 

 by treatment with iodine, since the water binding capacity is increased 

 (Jensen et at., 1950) and the rates of pepsin and trypsin digestion are de- 

 creased (Raghupathy et al., 1958). 



We have noted that 2 to 3 atoms of iodine are occasionally utilized for 

 each protein SH group. This might indicate (1) oxidation of SH beyond 

 the disulfide state, (2) reduction of the iodine beyond the iodide state, or 

 (3) some substitution of iodine in the cysteinyl residue. This problem was 

 studied by Fraenkel-Conrat (1955) with tobacco mosaic virus protein. It 

 is possible that sulfenate or sulfenyl iodide groups might be produced, but 

 it has always been thought that such groups are quite unstable and cannot 

 exist for appreciable time. However, the virus protein SH groups react with 

 2 atoms of iodine fairly rapidly, and this was shown to be accompanied by 

 the formation of sulfenyl iodide groups: 



V— SH + I2 -> V— SI + I- + H+ 



This group appears to be stable in this particular protein. Fraenkel-Conrat 

 pointed out that further reaction with thiols can form mixed disulfides: 



V— SH + R— SH ±^ V— S— S— R + H+ -f- I" 



Such reactions have been studied further in /5-lactoglobulin by Cunningham 

 and Nuenke (1959, 1960, 1961), using a spectrophotometric method. This 

 protein reacts with 4 equivalents of iodine to form 2 sulfenyl iodide groups 

 per mole of protein: 



P(— SH), + 2 I, -> P(— SI), + 2 I- + 2 H+ 



Ovalbumin reacts similarly but 6 equivalents of iodine are taken up. The 

 sulfenyl iodide groups are quite stable in these proteins, but can react with 

 simple thiols (e.g., glutathione, cysteine, and others) to form mixed disul- 



