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 oyster hemocytes. NADPH oxidase in vertebrate phagocytes generates superoxide anion from molecular oxygen and NADPH, which is then converted to additional 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 (10-500 microg/L) suppressed immunostimulated ROS and baseline NAD[P]H concentration but did not inhibit phagocytosis; the production of NADPH in stimulated cells was decreased only at the highest concentration. In this study, we evaluated the effects of TCIN (10-500 microg/L) on oyster hemocyte functions. As with striped bass macrophages, in vitro exposure to TCIN suppressed hemocyte ROS production in a dose-dependent manner, but did not affect phagocytosis. In contrast to the striped bass data, baseline NAD[P]H concentration was relatively unaffected and immunostimulated NAD[P]H production was marginally 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.