Phenoloxidase and cytotoxicity in the compound ascidian Botryllus schlosseri

The vacuoles of morula cells (MC) of the colonial ascidian Botryllus schlosseri contain phenoloxidase (PO). As the release of their vacuolar content at the border of incompatible contacting colonies is associated with the formation of necrotic masses which characterize the rejection reaction, the role of PO in Botryllus cytotoxicity was investigated. When hemocytes are incubated with blood plasma from incompatible (heterologous) colonies, MC degranulate and, after 60 min, the cytotoxicity index becomes significantly greater than that observed in controls incubated with autologous plasma. The rise in cell mortality is completely inhibited by the addition of PO inhibitors sodium benzoate, tropolone and phenylthiourea, and serine protease inhibitors phenylmethylsulfonyl fluoride, benzamidine, N-tosyl-L-phenylalanine chloromethyl ketone and N-tosyl-L-lysine chloromethyl ketone. The addition of either reducing agents L-cysteine and ascorbic acid or reactive oxygen species scavenger enzymes superoxide dismutase and catalase has a similar effect. Significant inhibition of cytotoxicity is also observed with the quinone scavenger, 3-methyl-2-benzothiazolinone hydrazone. In the presence of sodium benzoate and phenylthiourea, there is a significant reduction in the number, size and color intensity of necrotic masses along the contact border of incompatible colonies. A significant increase in superoxide anion production, completely inhibited by sodium benzoate, is observed when hemocytes are incubated with heterologous blood plasma. These results indicate that: (i) PO is the enzyme responsible for the cytotoxicity observed in both hemocyte cultures and rejection reactions; (ii) PO is present inside MC vacuoles as a proenzyme which is activated, upon release, by humoral proteases; (iii) cytotoxicity appears to be mainly due to oxidative stress generated by PO during oxidation of polyphenols to quinones without the involvement of other oxidases such as NADPH oxidase and peroxidase.