Effect of dysprosium on the spin-lattice relaxation time of cytochrome c and cytochrome a.

The progressive power saturation of the electron paramagnetic resonance of horse heart cytochrome c and solubilized bovine heart cytochrome oxidase has been monitored at low temperature in the presence of the relaxing agent, dysprosium. The saturation of the EPR signal of cytochrome c is relieved even at 6 K. With increasing temperature the effect is enhanced as the relaxation time of the dysprosium becomes shorter; however, the intrinsic spin-lattice relaxation time, T1, for cytochrome c decreases even more rapidly with increasing temperature. T1 for cytochrome c can be described by an intrinsic component, a component which is proportional to the concentration of dysprosium and a third component due to local binding which is independent of dysprosium concentration. The cytochrome a component of cytochrome oxidase is also affected by dysprosium. In the presence of cytochrome oxidase, T1 for cytochrome c is almost unaffected by dysprosium, indicating that access to the cytochrome c heme is blocked by the binding of c to oxidase. Based on the concentration-dependent effect of dysprosium on the lifetime of cytochrome c, it is possible to make distance estimates from the EPR active center to Dy3+. Dysprosium is therefore useful for determining the spatial relationships among paramagnetic enzyme components in a quantitative way.