Enzymology of Benzylisoquinoline Alkaloid Formation

Abstract

The largest group of naturally occurring alkaloids comprises compounds based on the tetrahydroisoquinoline nucleus. A multitude of structural types derived from this nucleus is found in the plant kingdom (Shamma 1972, Shamma and Moniot 1978). A great number of biosynthetic hypotheses based on in vivo investigations has been proposed to explain this unprecedented diversity (I.O. Spenser 1968, Shamma and Moniot 1978). The biosynthesis of isoquinolines at the cell-free level, however, has hardly been touched. Most of our knowledge on alkaloid biosynthesis is based on hypothetical pathways inferred from experiments in which radioactively labelled precursors and intact whole plants were used. Low rates of incorporation of the precursors into the products of interest have usually been observed, and channelling of these intermediates into abnormal pathways may also occur (e.g. Stöckigt 1980). Although in vivo techniques are necessary to provide first ideas about a biosynthetic route, they do not reveal the actual sequence and mechanism of the biosynthetic process in detail. Complex pathways can only be fully elucidated by a determination of the structures of enzymatically formed intermediates and by a characterization of the single enzymes or enzyme systems involved in these reactions. Enzymes of the secondary pathways can usually not be obtained easily from differentiated plants since the stationary levels of these enzymes appear to be very low and also the high content of phenolic compounds and other undesirable cell constituents tends to inactivate these enzymes during the isolation procedure (Loomis and Battaile 1966).