Distribution of mitochondrial calcium: pyroantimonate precipitation and atomic absorption spectroscopy

Mitochondrial function is highly dependent upon the concentrations of cations within the various compartments of this organelle. Utilizing variants of the pyroantimonate precipitation method, we have undertaken to localize cations, particularly calcium, in isolated mitochondria. Precipitated cations were present in two major compartments: 1) within the intracristal space and 2) in the matrix, frequently in association with the matrix face of cristal membranes. Precipitated cations were abundant in mitochondrial samples fixed with most pyroantimonate-containing fixatives, but were usually absent in samples fixed in phosphate-buffered, pyroantimonate-containing fixative solutions. Atomic absorption spectroscopy demonstrated that Ca2+ added to mitochondrial suspensions prior to fixation was retained in the mitochondrial pellet if 0.1 M collidine was present as the fixative buffer. When the same concentration of phosphate-buffered pyroantimonate-osmium fixative was employed, however, much of the added Ca2+ was lost to the supernatant. The suborganellar localization of cations was influenced by the presence of added respiratory substrate. Incubation of mitochondria in an isoosmotic buffered sucrose medium without succinate favored localization of precipitates in the intracristal space of the outer compartment. In the presence of succinate, the localization of precipitates was shifted toward the matrix compartment. This redistribution was particularly obvious in samples to which Ca2+ had been added and was blocked by the addition of antimycin. The observation of precipitated cations in the outer mitochondrial compartment supports previous evidence for a calcium-binding protein in this compartment.