The effects of chlorothalonil on oyster hemocyte activation: Phagocytosis,reduced pyridine nucleotides, and reactive oxygen species production

The production of reactive oxygen species (ROS) by a putative NADPH oxidase -like enzyme system is thought to contribute to antimicrobial activity in o yster hemocytes. NADPH oxidase in vertebrate phagocytes generates superoxid e anion from molecular oxygen and NADPH, which is then converted to additio nal ROS, including H2O2 and HOCl. The fungicide chlorothalonil (TCIN) is a thiol-reactive compound that binds to protein sulfhydryl groups, which can result in enzyme inactivation. NADPH oxidase, containing several sulfhydryl groups, is a potential target of TCIN. Previous studies have demonstrated that in vitro exposure of fish (Morone saxatilus) macrophages to TCIN (10-5 00 mu g/L) suppressed immunostimulated ROS and baseline NAD[P]H concentrati on but did not inhibit phagocytosis; the production of NADPH in stimulated cells was decreased only at the highest concentration. In this study, we ev aluated the effects of TCIN (10-500 mu g/L) on oyster hemocyte functions. A s with striped bass macrophages, in vitro exposure to TCIN suppressed hemoc yte ROS production in a dose-dependent manner, but did not affect phagocyto sis. In contrast to the striped bass data, baseline NAD[P]H concentration w as relatively unaffected and immunostimulated NAD[P]H production was margin ally suppressed at the higher exposure concentrations. Despite these minor differences, these results suggest that TCIN may also be inhibiting an NAD[ P]H oxidase-like enzyme in oyster hemocytes. (C) 2000 Academic Press.