Redox dependence of papain enzyme activity on charged kaolinite surfaces and in solution

The relative activity of an SH dependent enzyme, papain, is decreased by increasing the ratio of oxidizing disulfide to reducing thiol in solution. The same applies to papain adsorbed on negatively charged clay particles, but the effect is more pronounced if positively charged disulfides can be formed in the thiol-disulfide exchange equilibrium. Disulfides with a double positive charge are apparently more strongly attracted to clay than are singly charged, neutral, or negatively charged products of exchange. Preferential attraction of oxidizing disulfides to the particulate surfaces increases local, microenvironmental oxidizing potentials.