The energy metabolism of the intestinal parasite, Giardia lamblia, involves the iron-sulphur protein, pyruvate:ferredoxin oxidoreductase. Cell fractionation studies showed that this enzyme is associated with the membranes. NADH and NADPH dehydrogenases were found in both the membrane and cytosolic fractions. EPR spectroscopic studies showed the presence of iron-sulphur clusters in the membrane fraction and in the cytosolic fraction, non-sedimentable at 6 × 106 g · min. An acidic, soluble protein fraction was separated from the cytosol. It had an EPR spectrum in the reduced state, characteristic of the 2[4Fe-4S] type of ferredoxin, with g-factors at 2.04, 1.93 and l .89. and the midpoint redox potential was estimated to be −360 mV. This species is probably a ferredoxin, like those of anaerobic bacteria such as Closlridium and Desiilfovihrio spp. and also that of Entamoeba histolytica. The protein was readily and irreversibly oxidized to give [3Fe 4S] clusters.
