Mitochondrial matrix granules: their behavior during changing metabolic situations and their relationship to contact sites between inner and outer mitochondrial membranes.

Since their discovery in the early fifties mitochondrial granules have been the subject of many researches. Some twenty years ago two hypotheses on their function were introduced. Peachey thought that the granules were a sink of cations and that they would eventually regulate the concentrations of these ions. Alternatively, Barnard thought that the granules were precursors of the mitochondrial inner membrane. There are only a few data on organic constituents of the granules. Phospholipids (e.g., cardiolipin) glycoprotein or lipids, calcium precipitable lipoprotein, cytochrome c oxidase seem to be present in the granules. There has been much debate on whether calcium is present or not. Reports are mostly based on X-ray microanalysis, the result of which depends on preparation techniques. In heart muscle in stimulating situations the NMG (native matrix granules) move towards the inner membrane and are incorporated in it. They appear to create contact sites between inner and outer mitochondrial membranes in which enzymes can function efficiently. It is hypothetized that the system, NMG-contact sites, forms the structural basis of a regulatory mechanism, by which cells can cope with a high and sudden energy demand.

Freeze-fracture study of heart mitochondria in the condensed or orthodox state.

Rat heart mitochondria were isolated and forced in a well-defined metabolic state. After freeze-fracturing, the intramembrane particle dimension and density on both fracture faces of the inner mitochondrial membrane were measured. No significant differences could be calculated between the diameter of the membrane particles in the five different states. However, the particle density on the fracture faces of the inner mitochondrial membrane in the condensed configuration is significantly smaller than in the orthodox configuration on the 99.5% level of confidence. These results are compared with the literature, where conflicting data have been published about these particle densities.

Immunocytochemical investigation of native matrix granules of the rat heart mitochondrion.

We have examined the ultrastructure and the protein content of native matrix granules (NMG) in rat heart mitochondria, by postembedding immunocytochemistry. Cytochrome c oxidase was found to be present in these granules. It is believed that these granules contain incomplete inner mitochondrial membrane fractions, which can be incorporated in the membrane after stimulation of the metabolism.