Isolation and characterization of active N‐terminal truncated apo‐ and holoenzyme of mammalian liver tyrosine aminotransferase

Abstract

Limited proteolysis was used to probe the structure of the apo- and holoenzyme of rat liver tyrosine aminotransferase. Both were subjected to trypsinolysis and the major fragments were isolated and characterized. Trypsin cleaves the apoenzyme after residues Arg⁵⁷, Lys⁶⁴, and Lys⁷¹ and the holoenzyme after Arg³⁷ and Lys³⁸. The difference in the accessibility of the enzyme deprived or associated with pyridoxal 5′-phosphate reflects two distinct conformations. The activity, the affinity for the ligands and the thermostability of the purified truncated enzyme forms are similar to those of the native apo- and holoenzyme. A model for the domain structure of mammalian tyrosine aminotransferase and a mechanism for its rapid turnover are proposed.