Activation of serum prophenoloxidase in arthropod immunity. The specificity of cell wall glucan activation and activation by purified fungal glycoproteins of crayfish phenoloxidase.

This study shows that the activation of crayfish serum prophenoloxidase by carbohydrates was specific for beta-1,3-glucans. Fractionation of the beta-1,3-glucan laminaran into laminaran M and laminaran G showed that both activated the proenzyme, but the G-chain had somewhat higher affinity for the proenzyme. Methylation analysis of these two fractions revealed that there were no 1,6-linkages present. Laminaripentaose, a linear pentasaccharide composed of (1 leads to 3)-linked beta-D-glucopyranosyl residues was also active but had a lower affinity for the proenzyme than laminaran G. Laminaran completely inhibited the activation of prophenoloxidase by the pentaose. In the concentrations tested, laminaran was not inhibitory to the phenoloxidase activity. Purified extracellular glycoproteins of the parasitic fungus Aphanomyces astaci also strongly activated crayfish serum prophenoloxidase. Only high molecular weight glycoproteins were effective. Exo-beta-1,3-glucanase treatment decreased the activating capacity, suggesting that at least part of the glycoproteins consisted of beta-1,3-glucans. The significance of these results in the defence against parasitic fungi in crayfish is discussed.