Mitochondria, experimental

Hymenolepis diminuta. biochemical properties of peroxidase activity in mitochondria. Experimental Parasitology, 45 169-74. 

Rothman, A. H. (1968). Peroxidase in platyhelminth cuticular mitochondria. Experimental Parasitology, 23 51-5. 

It is possible to establish heterogeneous populations of mitochondria experimentally in the same cell. In human-mouse somatic cell hybrids, the mitochondria from the two parental types can be distinguished by differences in bouyant density of their DNA s (Attardi and Attardi, 1972) or differences in their nucleotide sequences (Coon et al., 1973). In human-mouse hybrid cells, Attardi and Attardi (1972) have failed to find persistence of human mitochondria, but Coon et al. (1973) reported... 

The chemiosmotic model requires that flow of electrons through the electron-transport chain leads to extrusion of protons from the mitochondrion, thus generating the proton electrochemical-potential gradient. Measurements of the number of H+ ions extruded per O atom reduced by complex IV of the electron-transport chain (the H+/0 ratio) are experimentally important because the ratio can be used to test the validity of mechanistic models of proton translocation (Sec. 14.6). 

Protoiis are extruded from the mitochondrion at the sites of NADH dehydrogenase, at the cytochrome b/c complex, and at cytochrome c oxidase. The exact number of protons driven out per electron at each of these steps has been difficult to determine. The ratio can vary under different experimental conditions. Extrusion of a proton at any particular step does not seem to be tightly coupled to the passage of an electron down the respiratory chain however, it is generally accepted that the passage of two electrons down the entire respiratory chain results in the translocation of 12 protons. 

Experimental data offered as evidence for hydrophobic bonding of lipids to protein by Green and Tzagaloff (1966) can be readily given interpretations which exclude hydrophobic bond involvement of the protein with lipid chains. For example, (1) the stronger binding to mitochondrion structural protein of lipids with longer acyl chains... 

From a biophysical point of view, the use of whole mitochondrion (or chloroplast) to test the "molecular" theory of Mitchell does not seem satisfying. The translocation of ions, such as protons, across a complex system, such as a cristae, is in itself ill-defined. The interactions between various fluxes (ion and water movements, electron and hole transport, etc.) are far too complex to be amenable to a simple analysis. At the present time, direct tests with the mitochondrial membranes are difficult. Experimental testing of the chemiosmotic hypothesis, using simpler model systems such as planar BLM and spherical liposomes are, therefore, in order. 

These facts bring us to one of the major outstanding problems in mitochondrial biogenesis what is the mechanism whereby the vast majority of mitochondrial proteins, coded for by nuclear DNA and synthesized on cytoplasmic ribosomes, traverse the mitochondrial membrane barriers and enter the closed organelle Since the mitochondrion is impermeable to proteins of even relatively low molecular weight, a number of models have been proposed to account for the transport of products of cytoplasmic protein synthesis into mitochondria, but convincing experimental support for these is not yet available. 

---