Abstract:
The prophenoloxidase (proPO) system is considered an important mechanism
of innate defence in arthropods. This enzymatic cascade has been studied in
crustaceans such as the crayfishes Astacus astacus and Pacifastacus leniusculus
and is located inside the haemocytes. An initial characterisation of this
system in the commercially important red swamp crayfish Procambarus clarkii
is described. The P. clarkii proPO system was activated by trypsin and also by
zymosan A. This activation was calcium ion dependent. The calcium ion
concentration also a#ected the background activation of the system and at
5 mM was highest as measured by the ability of phenoloxidase to oxidise
L-3,4-dihydroxyphenylalanine. The e#ect of calcium ions appears to be related
to the activation of an endogenous serine protease, but other calcium
ion-dependent factors can also a#ect proPO activation. Lipopolysaccharides
(LPS), glycolipids found in the outer leaflet of the outer membrane of
Gram-negative bacteria, were also able to activate the proPO system in P.
clarkii after a significant lag time of 25 to 30 min. However, LPS derivatives
(deacylated LPS, lipid A and â-D-GlcNAc-[1]6]-D-GlcNAc) were not able to
activate the enzymatic cascade in P. clarkii. Activation of the proPO system in
other crayfishes by LPS has been shown to be mediated by serine protease-like
enzymes. The observed e#ect of LPS and LPS derivatives on the activation of
the P. clarkii proPO system suggests that a protease activity triggered by
these molecules may be mediated through the recognition of a ‘‘complete’’ LPS
molecule (polysaccharide and lipid A). The intermolecular recognition of LPS
by a putative endogenous serine protease zymogen might explain the lag time
observed in proPO activation.
