The action of digitonin on rat liver mitochondria. Electron microscopy

1. Rat liver mitochondria were examined in the electron microscope by using negative staining in the presence of 0.3m-sucrose. The intact outer membrane does not appear to be freely permeable to the stain. Where the stain penetrated through a tear it was seen that the inner membrane had randomly oriented grooves, many of which contained round structures varying between 200 and 900å in diameter. Laminar structures containing two to five layers of approx. 50å each were found at the periphery. 2. When the outer membrane was removed by treating the mitochondria with digitonin several types of inner-membrane complexes were formed and they showed a general correlation with those observed in sectioned samples of the same preparations. The main types were: (a) a condensed form looking very much like the intact mitochondrion without the outer membrane (this still showed the grooves, some of which contained the round structures, and the laminar whirls at the edges); (b) a more transparent form containing tubules of uniform width and various lengths (some of these appeared to terminate in a hole at the surface of the inner membrane); (c) a large torn sac, probably the inner membrane, containing some tubules and vesicles. 3. When the inner-membrane complex was further treated with digitonin it was disrupted and the resulting material consisted of pieces of membrane, doughnut-shaped units and lamellar structures. Most of these pieces varied in size between 500 and 1000å.

The action of digitonin on rat liver mitochondria. The effects on enzyme content

1. The effects of repetitive treatment of rat liver mitochondria with digitonin were examined. The first treatment results in the removal of the outer membrane. Almost all the NADH-cytochrome c reductase (rotenone-insensitive) is lost whereas the major portions of the soluble and bound enzymes are retained. One exception appears to be the cytochromes, which undergo somewhat larger losses. The resulting inner-membrane complex carries out oxidative phosphorylation and P(i)-ATP exchange. 2. The properties of the inner-membrane complex are affected by the osmoticity of the medium. When it is suspended in water little protein is lost but there is a marked loss of phosphorylation. If after the suspension in water the particulate fraction is reisolated by centrifugation and treated with digitonin, or if the aqueous suspension is treated directly with digitonin and the particulate fraction then reisolated, the phosphorylation is largely restored. 3. Additional treatment of the inner mitochondrial complex with digitonin results in the formation of a particulate fraction that contains approx. 8% of the initial mitochondrial protein, no outer membrane, no soluble mitochondrial enzymes and is still capable of coupled oxidative phosphorylation and P(i)-ATP exchange. These effects cannot be reproduced by treatment with water. 4. The rat liver mitochondria and all of the resulting preparations obtained after digitonin treatment may be stored for long periods in dimethyl sulphoxide with little change of activity.

The action of digitonin on rat liver mitochondria. Phospholipid content

1. The amount and types of phospholipid and the fatty acid composition of the various phospholipids were examined in intact rat liver mitochondria, in mitochondria devoid of their outer membrane (preparation A) and in very small pieces derived from the disruption of the inner-membrane complexes (preparation B). The latter two preparations were obtained by digitonin treatment and carry out oxidative phosphorylation. 2. The ratio mug.atoms of phospholipid P/mg. of protein was 0.163 for intact mitochondria, decreased to 0.118 on removal of the outer membrane and increased markedly to 0.292 on disruption of the inner-membrane complex. 3. Examination of the various types of phospholipid present showed that the molar proportions cardiolipin:phosphatidylcholine:phosphatidylethanolamine were approx. 1:6:6 for intact mitochondria and 1:3:3 for preparations A and B. 4. There was a correlation between the recovery of cardiolipin and adenosine triphosphatase activity in the conversion of intact mitochondria into preparations A and B. 5. The fatty acid contents of the various types of phospholipid purified by thin-layer chromatography were identical in all three preparations. Our results show a considerably higher content of arachidonic acid and lower content of oleic acid for phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol than have previously been reported for mitochondrial phospholipids.

Studies on the role of the morphological constituents of the microsome fraction from rat liver in protein synthesis

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