The reactivity of the three disulphide bridges of insulin towards sodium sulphite was studied by amperometric titration of the liberated thiol groups. In the native, acetylated or succinylated molecule two bridges react at pH7, but in the methylated or phenylcarbamoylated molecule only one bridge reacts. All three bridges react in all derivatives in 8m-urea or at pH9. Loss in biological activity parallels the loss in reactivity of one of the bridges during methylation. It is suggested that change in reactivity of the S·S bonds reflects the occurrence of a conformational modification of the protein. The possibility is discussed that the unusually high reactivity of the S·S bonds in native insulin depends strictly on the integrity of the native molecule, suggesting that S·S bonds are in some way involved in the hormone9s mode of action.
