EFFECT OF pH ON THE REACTIVITY

It was reported in the previous paper that thiamine and O-benzoylthiamine (OBT) reacted with iodine-oxidized egg albumin resulting in formation of protein thiamine mixed disulfide (1). The formation of protein-bound thiamine is de pendent on the pH of the reaction mixture and the amount of bound thiamine was much greater at pH 9.0 than at pH 7.3 possibly due to the difference of the amount of the thiol form of thiamine which really took part in the reaction. Further, the reactivity of thiamine was different from that of OBT at the same pH levels (7.3 and 9.0), the latter being more reactive with protein than the former, In the present experiment, the reactivity at various pH levels was investigated for further confirmation, It was found that the formation of bound thiamine increased gradually from pH around 6, but rapidly at pH around 8.0, showing the similar tendency of the relationship between pH and the formation of thiol-type thiamine. OBT was confirmed to be more reactive with protein than thiamine at various pH levels. Therefore, the equilibrium concentrations of the thiol-type thiamine and OBT were compared by measuring the anodic wave height of polarography at constant pH levels and it was found that the anodic wave height of OBT was remarkably higher than that of thiamine at pH 7 to 8, showing the greater production of the thiol form. Thus the main factor of the difference in the reactivity of both the compounds with oxidized egg albumin was confirmed.