Wilhelm BOLAND, Martin VEITH and Neil. J. OLDHAM
Institut für Organische Chemie und Biochemie der Universität Bonn,
Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany.
According to recent investigations into iridoid biosynthesis in chrysomelides using deuterium labelled precursors, the methylcyclopentanoids are produced From geraniol, via 8-hydroxygeraniol, following a pathway similar to that known from plants [1]. Interestingly, all the enzymes required for oxidation of 8-hydroxygeraniol, the cyclisation of the acyclic dialdehyde and the final modifications of the cyclopentanoids are presented the biphasic liquid of the glandular reservoir of the larvae [2]. In vitro experiments with enantiospecifically labelled dials indicate that, among leaf beetles, an oxygen dependent oxidase specifically removes the prom hydrogen atoms from both ends of the diol precursor yielding 8-oxogeraniol as an acyclic intermediate.
Dependent on the insect larva, the dialdehyde can be cyclised to chrysomelidial by two different modes: 1. via plagiodial and subsequent isomerisation of the ring double bond (with loss of a hydrogen atom from C(4) of the acyclic precursor; most leaf beetle larvae), 2. directly to chysomelidial without loss of hydrogen atom from C(4) (e.g. Gastrophysa spp.) [1] Chirally labelled precursors (at C(4)) proof that the formation of plagiodial generally proceeds enantiospecifically with loss of the C(4)Hs deuterium atom.
The oxidase has a remarkably broad substrate tolerance which enables the enzyme to convert, besides various terpenoids, even aromatic precursors like benzylalkohol or o-hydroxybenzylalcohol into benzaldehyde or salicylaldehyde following the same principal. stereochemical course (Re-specificity) [3]. ln contrast to the corresponding plant enzyme, an NADPH dependent oxido-reductase [4], the larval enzyme requires molecular oxygen as the cofactor [2,3].
It appears likely that the broad substrate tolerance of the enzymes is the platform for an evolutionary development leading from archetype iridoids to "modern" defenses based on e.g. salicylaldehyde by simply changing the host plant and subsequent adaptation and exploitation of the peculiar biochemistry of the new host [5].