Branched electron transport chain in Trypanosoma mega

Workshop no. 1. Alternate metabolic pathways in protozoan energy metabolism

The purpose of this workshop was to collect together colleagues investigating the intermediary metabolism of protozoa, with a view to discussing those pathways involved in energy metabolism and the production of ATP and other high-energy compounds, together with the factors affecting energy balance. The aspects of energy metabolism chosen for discussion comprised the metabolic pathways ranging from the strictly anaerobic to highly oxidative; subcellular compartmentation of these pathways within the protozoa; the functional role of these pathways including a consideration of aero-tolerance; and the use of inhibitors as biochemical probes and potential chemotherapeuticagents. Hopefully this approach has produced a broad 'over-view' of important areas of protozoan energy metabolism which will enable both the specialist and non-specialist to appreciate the similarities and differences between the metabolic behaviour of a range of protozoa.

Terminal oxidases in the trypanosomatid Trypanosoma cruzi

Titration of Trypanosoma cruzi respiration with cyanide, with results treated as Dixon plots, indicated the presence of several terminal oxidases. The inhibitions obtained at low cyanide concentrations (0-300 microM), taken together with cyanide effects on cytochrome aa3-deficient, dyskinetoplastic epimastigotes, supported cytochrome aa3 as T. cruzi main terminal oxidase. By increasing cyanide concentration to 1.0 mM, two alternative terminal oxidases could be detected. One of these was active in both kinetoplastic and dyskinetoplastic (cytochrome aa3-deficient) epimastigotes, and azide- and antimycin-insensitive. Complementary cytochrome studies with intact epimastigotes and mitochondrial membranes revealed the presence of cytochromes aa3, b, c558, o and possibly d, as components of the parasite electron transport system. Fractionation studies demonstrated that both o and d were bound to the mitochondrial membrane. Reduction by endogenous substrates and complex formation with cyanide supported cytochrome o as alternative terminal oxidase. EB-cultured, dyskinetoplastic epimastigotes showed the same respiration rate as the kinetoplastic cells, despite the significant decrease of cytochrome aa3, thus indicating adaptive mechanisms that determine the expression of alternative oxidases, whenever the main terminal activity is depressed.

The sequence of the gene for cytochrome c oxidase subunit I, a frameshift containing gene for cytochrome c oxidase subunit II and seven unassigned reading frames in Trypanosoma brucei mitochondrial maxi-circle DNA

A 9.2 kb segment of the maxi-circle of Trypanosoma brucei mitochondrial DNA contains the genes for cytochrome c oxidase subunits I and II (coxI and coxII) and seven Unassigned Reading Frames ("URFs"). The genes for coxI and coxII display considerable homology at the aminoacid level (38 and 25%, respectively) to the corresponding genes in fungal and mammalian mtDNA, the only striking point of divergence being an unusually high cysteine content (about 4.5%). The reading frame coding for cytochrome c oxidase subunit II is discontinuous: the C-terminal portion of about 40 aminoacids, is present in the DNA-sequence in a -1 reading frame with respect to the N-terminal moiety. URF5, 8 and 10, show a low but distinct homology (about 20%) to mammalian mitochondrial URF-1, 4 and 5, respectively. In URF5, the first AUG is found at codon 145, whereas extensive homology to mammalian URF-1 sequences occurs upstream of this position. The possibility exists that UUG can serve as an initiator codon. URF7 and URF9 have a highly unusual aminoacid composition and do not possess AUG or UUG initiator codons. These URFs probably do not have a protein-coding function. The segment does not contain conventional tRNA genes.

Effect of inhibitors of electron transport and oxidative phosphorylation on Trypanosoma cruzi respiration and growth

Antimycin A and 2-heptyl-4-hydroxyquinoline N-oxide, two specific inhibitors of the b-c1 segment of the respiratory chain, affected the respiration of Trypanosoma cruzi epimastigote forms. The half-maximum inhibitory concentrations were about 0.05 and 4.0 micrograms/mg cells (dry wt.), respectively. The maximum effect of antimycin (about 80% inhibition of respiration) was at about 0.1 microgram antimycin/mg cells. Differential spectrophotometry of T. cruzi epimastigotes in the presence of antimycin, cyanide (or sulfide) and uncouplers, revealed the presence of functional cytochromes aa3, b and c558. In the stationary growth phase respiration by T. cruzi was completely inhibited by cyanide and effectively inhibited by sulfide, but in the exponential growth phase respiration was about 20% insensitive to 5 mM cyanide. Cyanide- and antimycin-insensitive respiration was completely inhibited by salicylhydroxamic acid (2 mM). Antimycin inhibited the operation of the tricarboxylic acids cycle in T. cruzi, as shown by the lesser production of 14CO2 and by the modification of 14C distribution in epimastigotes incubated with [1-14C]glucose, [2-14C]acetate or NaH14CO3. The inhibition of electron transport by antimycin increased the rate of the fumarate reductase reaction, an alternative electron pathway for the oxidation of reduced pyridine nucleotides. Addition of carbonyl cyanide 3-chlorophenylhydrazone to epimastigotes increased the rate of respiration and promoted the oxidation of reduced cytochrome b components, thus showing that these components are subject to respiratory (acceptor) control. Pentachlorophenol similarly affected the cytochrome b redox level but did not modify the rate of respiration. The uncouplers released N,N'-dicyclohexylcarbodiimide inhibition of respiration, and uncouplers and cyanide significantly decreased the ATP level in epimastigotes. The combined effects of the assay inhibitors on respiration, cytochrome b redox level, ATP content and energy charge confirmed the operation of oxidative phosphorylation in T. cruzi epimastigotes. Antimycin, uncouplers and N,N'-dicyclohexylcarbodiimide inhibited growth of T. cruzi, thus proving the essential role of oxidative phosphorylation for the parasite.

Evidence for a branched electron transport chain in Trypanosoma brucei

The flow of electrons the terminal oxidases present in the bloodstream and procyclic trypomastigotes of Trypanosoma brucei LUMP 1026 has been investigated by the use of salicylhydroxamic acid (SHAM) and cyanide. Respiration in bloodstream trypomastigotes was completely inhibited by 0.5 mM SHAM with a Ki below 10 microM. The Ki for SHAM in procyclic trypomastigotes was 70 microM. In procyclic trypomastigotes there are at least three terminal oxidases of which the two major ones are cytochrome aa3 oxidase, sensitive to cyanide inhibition, and alpha-glycerophosphate oxidase (GPO), sensitive to SHAM inhibition. These two oxidases contribute 60 and 30%, respectively, to total cell respiration. Inhibition of the cytochrome system with cyanide causes an increase in the flow of electrons through the GPO system, and inhibition of the GPO system with SHAM stimulates electron flow in the cytochrome system. Succinate oxidation in the mitochondrial fraction is partially inhibited by SHAM and this SHAM-sensitive respiration is not inhibited by antimycin A. The kinetic data of respiration by procyclic trypomastigotes fit a model proposed by Bahr and Bonner to determine the maximum rates of two competing electron transport pathways. It is concluded that the electron transport chain in T. brucei is branched.

Steady-state oxygen kinetics of terminal oxidases in Trypanosoma mega

Steady-state oxygen kinetics of Trypanosoma mega reveal the presence of 3 oxidases. These include an oxidase which is sensitive to salicylhydroxamic acid (SHAM) but insensitive to sodium azide. This oxidase could be the L-alpha glycerophosphate oxidase present in bloodstream trypanosomes. In addition, and oxidase is present wthich is azide-sensitive but SHAM-insensitive. This oxidase is inhibited by CO and is probably cytochrome aa3. A 3rd oxidase is insensitive to both azide and SHAM but is inhibited by CO and is possibly cytochrome o. Reciprocal plots of T. mega reveal the presence of 2 oxidases that are inhibited by CO. These results are discussed in the light of previous evidence suggesting the presence of several oxidases and a branched electron transport system in T. mega.

The cytochromes of Acanthamoeba castellanii

1. Low-temperature difference spectra of gradient-purified mitochondria of Acanthamoeba castellanii reveal the presence of cytochromes b-555, b-562 and c-549, with a-type cytochromes having a broad asymmetrical maximum at 602 nm; these components were also observed in specta of whole cells. 2. The a-type cytochromes are unusual in that they have split Soret absorption maxima (at 442 and 449 nm) and an uncharacteristic CO difference spectrum. 3. CO difference spectra of whole cells and 'microsomal' membranes show large amounts of cytochrome P-420 compared with cytochrome P-450. 4. Difference spectra in the presence of cyanide indicate the presence of an a-type cytochrome and two cyanide-reacting components, one of which may be cytochrome a3. 5. Whole-cell respiration in a N2/O2 (19:1) atmosphere was decreased by 50%, suggesting the presence of a low-affinity oxidase. This lowered respiration is inhibited by 50% by CO, and the inhibition is partially light-reversible; photochemical action spectra suggest that cytochrome a3 contributes to this release of inhibition. Other CO-reacting oxidases are also present. 6. The results are discussed with the view that cytochrome a3 is present in A. castellanii, but its identification in CO difference spectra is obscured by other component(s).

An alternate respiratory pathway in Candida albicans

Usual concentrations of antimycin A, rotenone and EDTA, individually or in combination, reduced aerobic growth rate and cell yield of Candida albicans to about half its normal level and to about the levels of previously-described acetate-negative, cytochrome-complete and aa3-deficient variants which were little affected by the inhibitors. Anaerobic conditions (not affected by antimycin A) reduced growth rate and cell yield of all cultures-including that of a nonrespiring aa3, b-deficient mutant-to low, equal levels. Antimycin A but not rotenone prevented growth of the normal strain on ethanol medium. Cyanide and antimycin A blocked most of the respiration of the normal strain and cytochrome-complete variant, but did not affect that of the cytochrome aa3-deficient mutant. Rotenone and EDTA did not affect respiration of any of the cultures. SHAM blocked cyanide-and antimycin A-insensitive respiration and prolonged the lag phases of the three respiring cultures, especially in the presence of antimycin A, but alone increased oxygen-uptake rate of the cytochrome-complete cultures while curtailing that of the cytochrome aa3-deficient mutant. Resting cells, especially wild-type, grown in medium containing antimycin A exhibited lowered oxygen-uptake rate, which was increased upon the addition of cyanide or antimycin A. Antimycin A stimulated, but cyanide inhibited, respiration of cytochrome-complete cultures grown in the presence of rotenone but did not affect that of the cytochrome aa3-deficient mutant. SHAM inhibited respiration of all antimycin A- or rotenone-grown cultures. The high rate of respiration of C. albicans in the presence of inhibitors for three sites of electron transport in the conventional oxidative pathway, the inhibition of this respiration by SHAM and its loss by the absence of cytochrome b, indicate an alternate oxidative pathway in this organism which crosses the conventional one at cytochrome b.