A Review of Electron Transport Mechanisms in Parasitic Protozoa

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Abstract
The literature related to electron transport in parasitic protozoa is reviewed. Plasmodia contain oxidases sensitive to cyanide, azide, and carbon monoxide, indicating the presence of cytochrome oxidase. Succinoxidase and other flavoproteins are present as judged by the reduction of indophenol and cresyl blue in oxygen uptake studies. The presence of intermediate carriers is inferred from the inhibition of oxygen uptake by naphthoquinones. Trichomonads have a limited capacity for oxygen utilization which is insensitive to cyanide and results in hydrogen peroxide accumulation except when catalase is also present. A flavoprotein terminal oxidase is indicated, but the importance of the oxygen utilization is not known. Some strains produce hydrogen gas which is probably linked to electron transport. The organisms are essentially anaerobic and therefore depend upon coupled reactions mediated by pyridino and flavoproteins resulting in the production of reduced compounds. Trypanosomes show a variety of electron transport systems linked to oxygen. Cyanide has for many years been used to distinguish two types a sensitive and an insensitive oxidase. In general the invertebrate or culture forms of all the trypanomsomes have cyanide sensitive respiration, whereas the vertebrate forms are of two types: the lewisi group are cyanide sensitive and the brucei group are insensitive. All groups show conventional glycolysis pathways, but only the sensitive group shows active citric cycle reactions and hemoprotein spectra. Little is known of the properties of the latter which seem to differ in some respects from analogous factors of mammals. The sensitive pathway to oxygen resembles that of mammals but the insensitive pathway has some unique characteristics. Reduced Nicotina-mide adenine dinucleotide (NADH)1 is reoxidized principally by di-hydroxyacetone phosphate to yield glycerophosphate. This is oxidized by a dehydrogenase which is probably a flavoprotein containing a sulfhydryl group and a metal linked to oxygen through an acetone soluble factor. Hydrogen peroxide is not found as it usually is in other cyanide insensitive oxidases.
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