On the Nature of the Dense Matrix Granules of Normal Mitochondria

A constant morphological feature of normal mitochondria, either isolated or in the intact tissue, is the presence of varying numbers of dense granules in the matrix. Isolated mitochondria contain relatively large amounts of Ca + + and Mg ++ (9) and are also able to accumulate divalent cations (1-3). Peachey (6) suggested that the normal matrix granules prime the accumulation of divalent cations, a conclusion that agrees with earlier morphologic interpretations of Rouillier (7). However, Vasington and Greenawalt (11, 12) and Greenawalt and Carafoli (5) found that the normal matrix granules are not the exclusive sites for the deposition of Ca ++ and Sr + by isolated mitochondria. Thus, the real nature and functional significance of the dense matrix granules in normal mitochondria are still unknown. It was previously shown (a) that most of the Ca ++ in mitochondria freshly isolated from normal rats is released by uncouplers of oxidative phcsphorylation (10) and (b) that 2,4-dinitrophenol (DNP) discharges the large electron-opaque granules formed when mitochondria accumulate Ca ++ and inorganic phosphate in vitro (4). In the present study, an attempt has been made to correlate the content of Ca + and Mg ++ with the number of normal matrix granules in mitochondria incubated for varying times in the presence and absence of DNP. The kinetics of release of Ca and Mg + and of the diminution in the numbers of matrix granules indicate that neither of these metals alone accounts for the presence or absence of matrix granules in normal mitochondria; the same is true for Mn + . However, the possible participation of a fairly large fraction of mitochondrial Mg + in the composition of the granules was not completely ruled out.