BIOCHEMICAL AND FUNCTIONAL DIFFERENCES IN RAT-LIVER MITOCHONDRIAL SUBPOPULATIONS OBTAINED AT DIFFERENT GRAVITATIONAL FORCES

Previous studies have reported that liver mitochondria may be fraction ated into different subpopulations. However, no careful studies have b een performed to exclude mitochondrial damage and to investigate more thoroughly the possible biochemical differences existing between the s ubpopulations. In this study, we analysed the integrity and the bioche mical properties of rat liver mitochondria. Mitochondrial fractions we re obtained by differential centrifugation at different gravitational forces: 1000 g (M1 fraction), 3000 g (M3 fraction) and 10,000 g (M10 f raction). The integrity of these organelles was checked by measuring c itrate synthase activity both in the presence and absence of Triton X- 100 detergent. Biochemical analyses included polarographic determinati on of cytochrome oxidase activity and respiratory parameters and spect rophotometric determination of cytochrome content. (1) The integrity o f mitochondria was almost homogeneous between fractions (88.5, 80 and 78.3% in M1, M3 and M10 fractions, respectively). (2) The heaviest M1 fraction contains mitochondria which are on average twice as large as M3 and about three times as large as M10. (3) The M1 fraction exhibite d the highest specific cytochrome oxidase activity (1040 +/- 20 nAtoms O/min x mg protein) and the highest respiratory rates (72 +/- 3 nAtom s O/min x mg protein and 526 +/- 45 nAtoms O/min x mg protein for Stat es 4 and 3, respectively). Oxidative capacity and respiratory rates de creased as the size of the organelles decreased, reaching values of 1/ 5 and 1/14 in the M3 and M10 fractions as compared to the M1. (4) Thes e changes are accompanied by a change in the respiratory control ratio (RCR), which varies from 7.3 in M1 to about 2.0 in M10. A similar tre nd was observed in cytochrome contents but the differences were not as great as cytochrome oxidase activity and State 3 respiration. These r esults, as a whole, show that a mitochondrial heterogeneity exists in rat liver cell. We suggest that the above-mentioned differences might represent steps of mitochondrial maturation. The maturation would be f undamentally based on the increase of efficiency of the mechanism for ATP synthesis.