Distant Precursors of Benzylisoquinoline Alkaloids and their Enzymatic Formation

The incorporation rates of labelled tyrosine, D O P A , tyramine, and dopamine have been inves­ tigated during the in vivo formation of the protoberberine alkaloid, jatrorrhizine, in callus cul­ tures o f Berberis canadensis. W hile tyrosine was equally well incorporated into both the isoquinoline (54% ) and benzyl (46% ) portions o f the alkaloid. D O PA was almost exclusively (91%) transformed into the isoquinoline m oiety. H owever, tyramine (25%) and to a lesser extent, dopamine (15% ) were incorporated into the aldehyde-derived, benzylic half of the isoquinoline m olecule as well. In order to investigate further the precursory roles of these compounds, select enzym es involved in tyrosine metabolism in alkaloid-producing cell cultures have been studied. The occurrence of tyrosine decarboxylase, phenolase, transaminase, p-hydroxyphenylpyruvate decarboxylase, amineoxidase and methionine adenosyl transferase was demonstrated in suspen­ sion cells of Berberis. These enzym es were partially purified and a preliminary characterization was performed. In the light of these and previous data, the differential metabolism of tyrosine and D O P A in the early steps of isoquinoline alkaloid biosynthesis is discussed. Conclusive evidence as to the biosynthetic origin o f the phenylacetaldehydes which furnish the benzylic m oiety of the alkaloids is precluded by the presence o f both amineoxidase and phenylpyruvate decarboxylase activities in these cultures.