The isolation of mitochondria from the amoeba Hartmanella castellanii Neff

Interactions between the cytochrome pathway and the alternative oxidase in isolated Acanthamoeba castellanii mitochondria

The steady-state activity of the two quinol-oxidizing pathways of Acanthamoeba castellanii mitochondria, the phosphorylating cytochrome pathway (i.e. the benzohydroxamate(BHAM)-resistant respiration in state 3) and the alternative oxidase (i.e. the KCN-resistant respiration), is shown to be fixed by ubiquinone (Q) pool redox state independently of the reducing substrate (succinate or exogenous reduced nicotinamide adenine dinucleotide (NADH)), indicating that the active Q pool is homogenous. For both pathways, activity increases with the Q reduction level (up to 80%). However, the cytochrome pathway respiration partially inhibited (about 50%) by myxothiazol decreases when the Q reduction level increases above 80%. The decrease can be explained by the Q cycle mechanism of complex III. It is also shown that BHAM has an influence on the relationship between the rate of ADP phosphorylation and the Q reduction level when alternative oxidase is active, and that KCN has an influence on the relationship between the alternative oxidase activity and the Q reduction level. These unexpected effects of BHAM and KCN observed at a given Q reduction level are likely due to functional connections between the two pathways activities or to protein-protein interaction.

Electron partitioning between the two branching quinol-oxidizing pathways in Acanthamoeba castellanii mitochondria during steady-state state 3 respiration

Amoeba mitochondria possess a respiratory chain with two quinol-oxidizing pathways: the cytochrome pathway and the cyanide-resistant alternative oxidase pathway. The ADP/O method, based on the non-phosphorylating property of alternative oxidase, was used to determine contributions of both pathways in overall state 3 respiration in the presence of GMP (an activator of the alternative oxidase in amoeba) and succinate as oxidizable substrate. This method involves pair measurements of ADP/O ratios plus and minus benzohydroxamate (an inhibitor of the alternative oxidase). The requirements of the method are listed and verified. When overall state 3 respiration was decreased by increasing concentrations of n-butyl malonate (a non-penetrating inhibitor of succinate uptake), the quinone reduction level declined. At the same time, the alternative pathway contribution decreased sharply and became negligible when quinone redox state was lower than 50%, whereas the cytochrome pathway contribution first increased and then passed through a maximum at a quinone redox state of 58% and sharply decreased at a lower level of quinone reduction. This study is the first attempt to examine the steady-state kinetics of the two quinol-oxidizing pathways when both are active and to describe electron partitioning between them when the steady-state rate of the quinone-reducing pathway is varied.

Immunological identification of the alternative oxidase of Acanthamoeba castellanii mitochondria

Mitochondria of the protozoa Acanthamoeba castellanii possess a cyanide-insensitive oxidase cross-reacting with monoclonal antibodies raised against the plant alternative oxidase. Immunoblotting revealed three monomeric forms (38, 35, and 32 kDa) and very low amounts of a single 65 kDa dimeric form. Cross-linking studies suggest that while in plants the alternative oxidase occurs as a dimer, in amoeba it functions as a monomer. Immunologically detectable protein levels change with the age of amoeba cell culture. Increased amounts of the 35 kDa protein are accompanied by an increase in the activity of cyanide-resistant respiration.

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).

Characterization of the genome of the small free-living amoeba Acanthamoeba castellanii

The cellular DNAs of Acanthamoeba castellanii have been characterized by their behaviour in CsC1 density gradients, by their thermal denaturation and by their renaturation kinetics. Whole-cell DNA exhibits, on CsC1 density gradients, a major peak with a density of 1.717 g/cm3 (major component) and a minor peak with a density of 1.692 g/cm3 (minor component). The major component is nuclear and the minor component is of cytoplasmic origin. The latter contains mitochondrial DNA as well as an extramitochondrial DNA fraction. Reiterated sequences make up approximately 14% of the total and are mainly cytoplasmic. They are characterized by three families of nucleotide sequences. The mitochondrial DNA exhibits a complex renaturation pattern. The fast renaturing component has a calculated complexity of 4.107 daltons. The slower renaturing component has a kinetic complexity tentatively estimated as 1.1010 daltons. The melting profile of mitochondrial DNA suggests heterogeneity in base composition.

The cytochromes of mitochondria from Tetrahymena pyriformis strain ST

1. Mitochondria of the obligately aerobic ciliate protozoon, Tetrahymena pyriformis strain ST, are unusual in that they possess a cytochrome oxidase system that does not react with reduced mammalian cytochrome c; the presence of cytochromes a(603)+a(3) is masked in the alpha-band region of spectra by the broad absorption band of cytochrome a(620). 2. Other haemoproteins present include cytochromes b(560), b(556), c(553) and c(549). 3. The reaction of reduced cytochrome a(3) with CO is reversed by flash photolysis, and in the presence of O(2) the subsequent oxidation of this cytochrome is followed by that of cytochrome a(603). 4. Cytochromes a(620) and b(560) also react with CO and with KCN; the latter cytochrome corresponds with that designated cytochrome o by other workers. 5. The contribution of cytochrome a(603) to difference spectra is revealed by making use of the fact that it does not react with KCN. 6. Cytochrome a(620) is unstable, and its alpha-absorption band is lost from spectra of mitochondria which have been aged or treated with ultrasound, detergents or organic solvents. 7. Possible pathways of electron transport via the several different terminal oxidases in Tetrahymena mitochondria are proposed.