Reversible inactivation of tyrosine aminotransferase from guinea pig liver by thiol and disulfide compounds.

Tyrosine aminotransferase from guinea pig liver is strongly inactivated by a variety of natural thiols and disulfides. L-cysteine was used as a model compound in the study of inactivation. Inactivation is due to the disulfide produced by spontaneous oxidation of thiol during incubation. Binding studies with ³⁵S]-cysteine revealed simultaneous incorporation of ³⁵S] into tyrosine aminotransferase and loss of enzyme activity. The reversibility demonstrates that the inactivation is the result of the formation of mixed disulfide between the disulfide and the sulfhydryl group of tyrosine aminotransferase. Some features of the enzyme active site are showed by the inactivation reaction.