Purification and characterization of tyrosine aminotransferase activities from Anchusa officinalis cell cultures

Three activities of tyrosine aminotransferase (TAT; EC 2.6.1.5), the enzyme which catalyzes the first step of the tyrosine pathway leading to the formation of rosmarinic acid (alpha-O-caffeoyl-3,4-dihydroxyphenyllactic acid), have been extensively purified from cell suspension cultures of Anchusa officinalis L. and subsequently characterized. TAT-1, TAT-2, and TAT-3 differ slightly in native molecular weights (180,000-220,000) and are composed of subunits (4 X 43,000 for TAT-1 and 4 X 56,000 for TAT-2). All three enzymes show a pronounced preference for L-tyrosine over other aromatic amino acids, but TAT-2 and TAT-3 can also effectively utilize L-aspartate or L-glutamate as a substrate. For amino acceptor cosubstrates, either oxaloacetate or alpha-ketoglutarate can be utilized equally well by TAT-1, while the former is the most effective alpha-keto acid for TAT-2 and the latter is the best for TAT-3. All the TAT activities display high pH optima (8.8-9.6), and are inhibited by the tyrosine metabolite 3,4-dihydroxyphenyllactate. TAT-2 and TAT-3 are also inhibited by rosmarinic acid.