1. Mitochondria isolated from livers of fed adult, starved adult, and embryonic rats can be separated into three distinct bands by isopycnic density centrifugation on a sucrose density gradient. The least dense band (B1) has a mean buoyant density of 1.162 and consists mainly of disrupted mitochondria. The middle band (B2) has a mean buoyant density of 1.184. The most dense band (B3) has a mean buoyant density of 1.216. B2 and B3 consist of intact mitochondria. 2. The mitochondria in B2 and B3 have very similar protein/phospholipid ratios, virtually identical phospholipid and fatty acid compositions and similar specific activities for cytochrome oxidase, malate dehydrogenase and monoamine oxidase. Both fractions have very low glucose 6-phosphatase and acid phosphatase activities. 3. As isolated, adult rat liver mitochondria have electron-dense matrices (condensed forms); some embryonic liver mitochondria are condensed, but a significant proportion have dilated matrices. All B2 mitochondria are in the condensed form. B3 mitochondria from adult rats are condensed if fixed in their equilibrium-density sucrose, but when this is diluted rapidly to 0.25m they become swollen. Some B3 mitochondria from embryonic rats are condensed, the others have dilated matrices. They all swell if rapidly diluted to 0.25m-sucrose. B2 mitochondria retain their condensed form on dilution of the sucrose. 4. It is concluded that the matrix space of B2 mitochondria is almost totally inaccessible to sucrose, but that of B3 mitochondria is readily accessible to sucrose. 5. In liver from normally fed adult rats the B2 mitochondria predominate, whereas in starved rats B2 and B3 are present in approximately equal proportions. Mitochondrial preparations from embryonic liver consist predominantly of B3 mitochondria, but the proportion of these decreases progressively as development proceeds. 6. The B2 mitochondria from livers of fed adult rats can be converted into B3 mitochondria by incubation with 10mm-succinate and 10mm-phosphate. 7. Some B2 mitochondria are converted into B3 mitochondria by exposure to high concentrations of sucrose.

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