Bovine lens leucine aminopeptidase. A study of possible roles for the thiol groups in holo- and apoenzyme.

The apoenzyme of leucine aminopeptidase has to be prepared either under inert conditions or in the presence of 2-thioethanol. Due to the removal of essential zinc ions, buried -SH groups are exposed. These -SH groups show a higher reactivity than the six -SH groups accessible in the native zinc enzyme. During the incubation of the zinc enzyme with guanidine hydrochloride, up to 36 -SH groups are titratble in dependence on the concentration of guanidine hydrochloride. The -SH groups can be distinguished by their reactivity towards Ellman's reagent and can be differentiated into four classes with respect to their function.