Characterization of the Activity of Tyrosinase on Betaxanthins Derived from(R)-Amino Acids

摘要

The activity of tyrosinase(EC 1.14.18.1)on selected(R)-betaxanthins is characterized in depth,demonstrating that the activity of the enzyme is not restricted to betaxanthins derived from(S)-amino acids.Conversion of(R)-tyrosine-betaxanthin [(R)-portulacaxanthin II] to the pigment(R)-dopaxanthin and its further oxidation to a series of products is described.Compound identity was studied by high performance liquid chromatography and electrospray ionization-mass spectrometry.The reaction rate on the(R)-isomer of dopaxanthin is 1.9-fold lower than that obtained for the(S)-isomer in previous studies.Tyrosinase showed stereospecificity in its affinity toward betaxanthins.The characterization of the activity of tyrosinase on(R)-betaxanthins reinforces the role of the enzyme in the biosynthetic scheme of betalains.