Metabolic and structural states of mitochondria. I. Regulation by adenosine diphosphate.

Mitochondria, the major site of synthesis for energy rich phosphate bonds in animal tissues, have been alluded to as the ionic pumps in cells because of the action of energy rich compounds in the control of osmotic properties (1, 2). Since the work of Raaflaub (3, 4) there have been several reports (1, 5-8) that adenosine triphosphate plus magnesium or manganous ions can reverse or prevent swelling of isolated mitochondria. Swelling and shrinking of mitochondria is also known to be accompanied by changes in intramitochondrial water and ions (1, 2, 7)) by changes in light scattering properties (3, 4, 9, 10) and by functional changes such as the ability to carry out oxidative phosphosphorylation (4, 7, 8, 10, 11). These findings suggest that a close relationship exists between mitochondrial size and metabolism. In 1955, Holton (12) reported that in respiring heart mitochondria the steady state of cytochrome b was more reduced in the presence of ADP. These findings conflicted with Chance and Baltsheffsky (13) who found cytochrome b more oxidized under these conditions. The discrepancy was studied by Chance and Packer (14) and was found due to the use of a single wave length measurement by Holton (12) in which the small optical changes of cytochrome b in the steady state were obscured by concurrent larger light scattering changes. These results indicated that mitochondrial swelling and shrinking changes accompanied the different metabolic states. Furthermore, since the intramitochondrial regulation of respiratory rate is largely under the control of ADP (Lardy and Wellman (15), Siekevitz and Potter (16)) Chance and Williams (17, 18))) it seemed reasonable that ADP rather than ATP was a more likely substance for regulating changes in mitochondrial size by metabolism. This hypothesis has been examined and confirmed by simultaneously monitoring swelling and shrinking during the different metabolic states of isolated mitochondria.’ In subsequent communications, the regulatory effect of phosphate and an account of the mechanism of this phenomenon in mitochondrial membrane fragments shall be reported (19).