614 6. ARSENICALS 



assuming a molecular weight of 44,000 for ovalbumin. If 1 molecule of 

 arsenical combines with 2 SH groups, this would indicate 4.8 reactive SH 

 groups on ovalbumin. Since there are 5 cysteine residue, this checks quite 



satisfactorily. It is not known if the arsenical reacts with 2 SH groups 

 on a single ovalbumin molecule or with SH groups on different molecules 

 (the figures above indicate at least some intermolecular linking). In the 

 experiments of Rosenthal, it is possible that some arsenical reacted with 

 the native proteins but split off during the dialysis. 



Stocken and Thompson (1946 a) studied the reactions of arsenicals 

 with kerateine (reduced keratin) and it was this work which originally led 

 the Oxford group to postulate the cyclic thioarsinite structure. Kerateine 

 was titrated with lewisite at pH 7 until the free SH groups had disappeared; 

 arsenite reacted less readily and a high concentration was required (even 

 then all the SH groups were not reacted). The arsenical-proteins are soluble 

 below pH 4 and above pH 5.4, exhibiting a minimum solubility around 

 pH 4.6. When these derived proteins were dialyzed, it was found that the 

 lewisite complex, is more stable: 20-hr dialysis caused a loss of 40% of 

 the bound lewisite and 85% of the bound arsenite, while 40-hr dialysis 

 gave a 53% loss of lewisite and a 100% loss of arsenite. Evidence that the 

 binding is primarily through SH groups was obtained by showing that 

 metakeratin (reoxidized kerateine) binds only about one tenth as much 

 arsenical as does kerateine, and this is easily removed by dialysis, indicating 

 nonspecific adsorption. Porphyrindin titrations carried out during the reac- 

 tion indicated that the arsenicals combine with two SH groups. It is not 

 known if a ring structure is formed or whether the binding is between 

 different kerateine molecules; the decreased solubility might possibly 



