The influence of osmolarity on the reduction of exogenous cytochrome c and permeability of the inner membrane of Jerusalem artichoke mitochondria

Abstract

The stimulation of succinate–cytochrome c reductase in Jerusalem artichoke mitochondria by lowering osmolarity was found to be associated with conformational changes in the inner membrane rather than with rupture of the outer membrane. This conclusion is based on the following evidence. (1) When the activation of succinate dehydrogenase was measured by using either K3Fe(CN)6 or exogenous cytochrome c as an electron acceptor, electron flow to cytochrome c was always 7% of that to K3Fe(CN)6 throughout the activation process. (2) The rate of exogenous cytochrome c reduction by succinate and NADH was directly related to the maximum rate of electron flow as determined by oxygen utilization. These two observations are not consistent with the low rate of succinate–cytochrome c reductase being limited by a permeability barrier at the outer membrane. (3) In addition to stimulating the succinate–cytochrome c reductase, lowering the osmolarity caused simultaneous changes in the permeability of the inner membrane to ferricyanide and NADH. The data show that lowering the osmolarity results in progressive changes in the permeability of the inner membrane. The first change detected was an increased permeability to K3Fe(CN)6, then a simultaneous increase in accessibility of the respiratory chain to exogenous cytochrome c and an increased permeability to NADH, followed finally by rupture as measured by the release of malate dehydrogenase.