The reactivities of tyrosine and tryptophan residues in lipid-bound cytochrome b5

A photo-chemically induced dynamic nuclear polarization NMR study on rabbit and bovine cytochrome b5

Although it has been indicated that proteins with chromophoric groups are not suitable for photo-chemically induced dynamic nuclear polarization (photo-CIDNP) measurements, we have successfully obtained these spectra for a heme protein, cytochrome b5. The characteristics of the spectra differed in some points from those so far reported. The intensities of the signals in the aromatic region were very weak, while those of the beta-methylene protons of one histidine and one tryptophan were extremely strong in comparison with the aromatic protons. It was demonstrated, on the basis of the photo-CIDNP spectrum, that one of seven histidines, all three tyrosines and a single tryptophan of the rabbit soluble cytochrome b5 are exposed on the surface of the protein. The results of comparison of the photo-CIDNP spectra for the rabbit soluble and intact, and bovine intact, cytochrome b5 led us to the conclusion that the conformation of the hydrophilic, catalytic part of cytochrome b5 is quite similar among these three proteins. In the presence of Chaps micelles, bovine intact cytochrome b5 was in monomeric form and the histidine signals disappeared from its photo-CIDNP spectrum. When bovine intact cytochrome b5 was reconstituted into egg yolk phosphatidylcholine liposomes, although separate signals due to the protein part were observed in the normal 1H-NMR spectrum, no photo-CIDNP signal could be detected. The normal spectrum suggests that the conformation of the protein embedded in liposomes is similar to that of the oligomeric form without lipids or a detergent.

The interaction of the membrane binding segment of cytochrome b5 with phospholipid vesicles. A pulse-radiolysis investigation

The reactions of the radical anions Br2- and (SCN)2- produced by pulse-radiolysis have been used to study the interaction of dipalmitoyl phosphatidyl choline vesicles with the membrane binding segment (MBS) of cytochrome b5. Tryptophan oxidation by Br2- at neutral pH was characterized spectrophotometrically in detergent-solubilized solutions of MBS; the attack of tyrosine by (SCN)2- under alkaline conditions could also be observed directly. The incorporation of MBS into the lipid bilayer protected the tryptophan, tyrosine and methionine residues from oxidation by Br2- or (SCN)2-. Some structural implications of these results are discussed.

Transfer of cytochrome b 5 and NADPH cytochrome c reductase between membranes

NADPH-cytochrome c reductase also reduces cytochrome b 5. The reduction is very slow when the proteins are in solution or bound to different membranes. Only when both proteins share a common membrane, is cytochrome b 5 reduced rapidly by NADPH. The difference in reaction rates indicates recombination on a common membrane of cytochrome b 5 and NADPH reductase originally bound to different vesicles. The recombination of the two proteins occurs with a variety of biological membranes (previously enriched with either reductase or cytochrome b 5) as well as with liposomes. We explain this process as protein transfer rather than vesicle fusion for several reasons: 1. The vesicles do not alter shape or size during incubation. 2. The rate of this process corresponds to the rate of incorporation of the single proteins into liposomes carrying the 'complementary' protein. 3. The exchange of proteins between biological membranes and liposomes occupied by protein does not change the density of either membrane. Protein transfer between membranes appears to be limited to those proteins which had spontaneously recombined with a preformed membrane. In contrast, proteins incorporated into liposomes by means of a detergent were not transferred, nor were endogenous cytochrome b 5 and NADPH-cytochrome c reductase transferred from microsomes to Golgi membranes or lipid vesicles. We conclude that the endogenous proteins and proteins incorporated in the presence of a detergent are linked to the membrane in another manner than the same proteins which had been inserted into a preformed membrane.