The superreactive disulfide bonds in alpha-lactalbumin and lysozyme

The disulfide reduction kinetics in equine lysozyme (ELZ), which is a Ca(2+)-binding lysozyme, and human (HLA) and equine alpha-lactalbumin (ELA) at pH 8.5 and 25 degrees C by excess dithiothreitol were studied, and it was found that in ELZ there is no superreactive disulfide bond, while one of the disulfides is reduced very quickly by the reducing agent in HLA and ELA, as in bovine alpha-lactalbumin. The local conformation around the surface disulfide in ELZ seems to be more similar to that in hen egg-white lysozyme than in alpha-lactalbumin. The four disulfides in ELZ were reduced slowly in an apparently single-exponential form, and the bound Ca2+ lowered the reduction rate. The torsion energy on each of the disulfides in three alpha-lactalbumin and eight c-type lysozymes whose native conformations have been experimentally or theoretically analyzed was calculated, and it was found that torsion imposed on the surface disulfide between Cys 6 and Cys 120 in alpha-lactalbumin is a main cause of the superreactivity and all of lysozymes, including the Ca(2+)-binding ones, have no such strained surface bond.