Motional dynamics of functional cytochrome c delivered by low pH fusion into the intermembrane space of intact mitochondria

Abstract

We have investigated the motional dynamics of cytochrome c in the intact, functional rat liver mitochondrion. To do this, functional, FITC-cytochrome c (fluorescein isothiocyanate monoderivatized cytochrome c) was incorporated into the intermembrane space (IMS) of intact mitochondria through encapsulation of cytochrome c into asolectin liposomes followed by low pH-induced fusion of the liposomes with the outer membranes of the mitochondria. A cytochrome c controlled enrichment of between 15%-50% (1800-7200 molecules incorporated per mitochondrion) was obtained. All cytochrome c incorporated, regardless of the quantity, participated in the function of electron transport, indicative of a functional, independent random diffusant. Resonance energy transfer was determined from the IMS-entrapped functional FITC-cytochrome c to octadecylrhodamine B incorporated into the mitochondrial membranes. Resonance energy transfer from FITC-cytochrome c to octadecylrhodamine B in isolated inner or outer mitochondrial membranes (IMM and OMM, respectively) was also measured. We found substantial differences in the effects of ionic strength (I) on the proximity of cytochrome c to isolated IMM and OMM. Interactions with isolated IMM were very dynamic, i.e., very I-dependent, and cytochrome c binding to IMM was significant only at very low I. I-dependent interactions of cytochrome c with isolated OMM were less I-dependent than those for the IMM. However, FITC-cytochrome c was essentially released from IMM and OMM at physiological I. The proximity of FITC-cytochrome c to each mitochondrial membrane after its incorporation into the IMS of intact mitochondria in the condensed configuration was estimated at different external, bulk I using: (a) resonance energy transfer from IMS-entrapped FITC-cytochrome c to octadecylrhodamine B-label evenly distributed in both mitochondrial membranes; and (b) resonance energy transfer from IMS-entrapped FITC-cytochrome c to octadecylrhodamine B-label concentrated in the OMM. Resonance energy transfer showed that the average distance between cytochrome c and the two IMS-membrane surfaces increased with increasing IMS-I, approaching a maximal measurable distance of 85 A at 150 mM I. This result is consistent with a dissociation of FITC-cytochrome c and both membranes of intact mitochondria at physiological I, i.e., when the activity of cytochrome c in electron transport is highest. Our findings reveal a primarily three-dimensional diffusion mode for IMS-cytochrome c during its function in electron transport in intact mitochondria at physiological I, and offer further evidence that mitochondrial electron transport is a process driven by random collisions between its independently diffusing electron transferring, redox components.