Wound activation of protoxins in marine sponge Aplysina aerophoba

The marine sponge Aplysina aerophoba accumulates brominated isoxazoline alkaloids, which include aerophobin-2, aplysinamisin-1, and isofistularin-3 as major constituents. Following disruption of compartmentalization, the isoxazoline alkaloids are enzymatically converted to aeroplysinin-1, which in rum gives rise to a dienone. The described bioconversions were demonstrated for the first time in vitro using an enzyme preparation from A. aerophoba. Cell-free extracts of other Aplysina species were capable of performing the same bioconversions, whereas enzyme extracts of sponges from other orders, which lack isoxazoline alkaloids, were inactive with regard to the reactions analyzed. These findings suggest that the enzyme activities studied are linked to the accumulation of suitable substrates and hence represent a specific biochemical property of sponges from the genus Aplysina. Time-course experiments with A. aerophoba, performed in situ, demonstrated that wound-induced bioconversions of isoxazoline alkaloids proceeded rapidly. Within 40 sec after mechanical damage of a tube of A. aerophoba, both aerophobin-2 and aplysinamisin-1 were completely converted to the dienone. The wound activation of protoxins results in a pronounced increase of the fish deterrent activity of A. aerophoba as shown in bioassays employing the common Caribbean wrasse Thalassoma bifasciatum.