“…The resistance of the protein thiols to reduction may therefore be due to both the charge and steric environment of the disulphide bonds. Racker (1955) and Thompson, Black & Hudson (1960) have shown that preparations of glutathione reductase contain an enzyme catalysing disulphideexchange reactions. The possibility exists, therefore, that the differences in the rates of reaction of the simple disulphides used are due not only to charge and steric effects but also to characteristics of this enzyme.…”