Flavohemoglobins are enzymes primarily implicated in nitrosative stress due to their high nitric oxide (NO) dioxygenase activity and transcriptional upregulation in response to NO. Giardia intestinalis assemblages A, B, and E possess flavohemoglobins (gFlHb) that may function beyond their NO dioxygenase activity, potentially contributing to oxidative stress regulation, as transcriptional profiling revealed that peroxide also induces gFlHb expression. This study investigates gFlHb's NADH oxidase activity in the absence of NO, structural interactions with lipids, and response to reactive oxygen species. Minor differences in NADH oxidase activity among assemblages were observed, and their susceptibilities to inhibition were assessed to evaluate gFlHb as a potential therapeutic target against Giardia infection. Under aerobic conditions, we observed that gFlHb generates hydrogen peroxide, a surprising finding suggesting a self-regulating feedback mechanism involving reactive oxygen species and heme degradation. These findings provide new insight into the role of flavohemoglobins in microaerotolerant parasites like Giardia.
Author Keywords: flavohemoglobin, Giardia intestinalis, heme, hydrogen peroxide, NADH, oxidative stress