Giardia intestinalis escapes oxidative stress by colonizing the small intestine: A molecular hypothesis

Abstract

SummaryGiardia intestinalis is the microaerophilic protozoon causing giardiasis, a common infectious intestinal disease. Giardia possesses an O 2 -scavenging activity likely essential for survival in the host. We report that Giardia trophozoites express the O 2 -detoxifying flavodiiron protein (FDP), detected by immunoblotting, and are able to reduce O 2 to H 2 O rapidly ( 3 lM O 2 3 min 3 10 6 cells at 37 8C) and with high affinity (C 50 5 3.4 6 0.7 lM O 2 ). Following a short-term (minutes) exposure to H 2 O 2 100 lM, the O 2 consumption by the parasites is irreversibly impaired, and the FDP undergoes a degradation, prevented by the proteasome-inhibitor MG132. Instead, H 2 O 2 does not cause degradation or inactivation of the isolated FDP. On the basis of the elevated susceptibility of Giardia to oxidative stress, we hypothesize that the parasite preferentially colonizes the small intestine since, compared with colon, it is characterized by a greater capacity for redox buffering and a lower propensity to oxidative stress.

A Detoxifying Oxygen Reductase in the Anaerobic Protozoan Entamoeba histolytica

“…However, there were no significant changes in EhFdp1 transcript abundance under a variety of conditions, including response to NO-derived stress (1, 2, 11, 13, 42). FDPs have been shown to be highly and constitutively expressed both in prokaryotes (in some cases incorporating an oxidative stress response operon [8,18]) and in protozoa (10,25,32). However, an increase in expression upon oxygen exposure has been observed for only one other FDP, that from the hyperthermophilic bacterium Thermotoga maritima, which shows an increase in protein levels with unmatched transcriptional levels and is proposed to sustain the growth of this "strict" anaerobe under microaerophilic conditions (20)(21)(22).…”

“…These genes were most probably acquired from prokaryotes by lateral gene transfer (3,4). So far, the protozoan enzymes have been shown to act as oxygen reductases (10,25,32).…”

Functional Characterization of Peroxiredoxins from the Human Protozoan Parasite Giardia intestinalis

“…Defense systems against O 2 , NO and superoxide anion (O 2 −• ) have been recently identified in Giardia as: a flavodiiron protein that, like a previously characterized NADH oxidase [12], is able to convert O 2 to H 2 O [13], [14]; an inducible flavohemoglobin able to aerobically metabolize NO to nitrate [15], [16] and, more recently, a superoxide reductase reducing O 2 −• to hydrogen peroxide (H 2 O 2 ) [17]. Identification of the H 2 O 2 -producing superoxide reductase is puzzling, because Giardia lacks catalase and no H 2 O 2 -metabolizing enzymes have been characterized in the parasite to date; a NADH peroxidase activity has been reported in membrane extracts of the parasite [5], but the enzyme responsible for this activity is as-yet unidentified.…”

Comparative biochemistry of Giardia, Hexamita and Spironucleus: Enigmatic diplomonads

“…Rich in flavoproteins [15,23], including presumably the flavodiiron protein terminal oxidase responsible for O 2 scavenging [32], the most prominent metabolic characteristic is the copious cyanide-, CO-, and metronidazole-sensitive generation of dihydrogen gas indicative of an Fe-only hydrogenase [33]. This occurs at rates 38-fold greater than that in G. intestinalis [34].…”