The dynamics and environment of water in suspensions of isolated rat liver mitochondria have been investigated by H NMR. NMR longitudinal and transversal relaxation times (T and T) were measured in the resuspension medium (2.65 s and 44.57 ms) and in mitochondrial suspensions (1.74 s and 23.14 ms), respectively. Results showed monoexponential relaxation in both cases, suggesting a fast water exchange across the inner mitochondrial membrane. Ferromagnetically induced shift of the extramitochondrial water with nonpermeant ferromagnetic particles revealed no detectable water signal from the intramitochondrial compartment, confirming the fast exchange case. Simulations on a two-compartment model indicated that the intramitochondrial water residence time has an upper limit of approximately 100 µs. Calculated intramitochondrial relaxation times revealed that the intramitochondrial environment has an apparent viscosity 30 times larger than the resuspension medium and 15 times larger than the cytosol of erythrocytes. The higher apparent viscosity of the mitochondrial matrix could account for reductions of more than one order of magnitude in the diffusion coefficient of water and other substrates, limitations in the rate of enzymatic reactions which are diffusion controlled and a more favorable formation of multienzyme complexes.

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