1Institut für Pharmazeutische Biologie der Technischen Universität, Mendelssohnstrasse 1, D-38106 Braunschweig, Germany.
2Laboratoire de Biologie Animale et Cellulaire, Université Libre de Bruxelles, B-1050 Bruxelles, Belgium.
Chrysomela populi and Phratora vittelinae are leaf beetles
(Chrysomelidae) which feed on Salix or Populus species. Their larvae
have exocrine defensive glands which produce a high concentrations of salicylaldehyde.
Previous studies had shown that the aldehyde is enzymatically produced from salicine
(salicylalcohol glucoside) by a sequential action of extracellular ß-glucosidase and
alcohol oxidase. Here we report on the kinetic properties and mechanism of the oxidase.
The enzyme was purified and identified to be a oxidase catalyzing the following reaction:
The enzymes from the two beetles species show a Mr of 80 000 in SDS-PAGE.
The genuine enzyme of Phratora was found to be a tetramer. The two enyzmes
display a high specificity for ortho-substituted benzylalcohols. Salicylalcohol is always the
best substrate. The enyzme from C. populi but not P. vittelinae also
oxidizes benzylalcohol. The Km-values are in the order of 100 mM. The
oxidase produces high concentrations of salicylaldehyde. This is kinetically possible because
of the different solubilities of the substrate (salicylalcohol: 74 g/l) and the product
(salicylaldehyde: 3 g/l). Salicylaldehyde is withdrawn from the water phase and builds up a
organic phase. The relation between the water phase (enzymatic phase) and the organic
phase (defensive phase) is 4 : 1 (v/v).