Comparison between the folding of reduced hen egg white lysozyme and that of reduced human milk lysozyme

In vitro, renaturation of reduced and unfolded lysozyme is catalyzed by a mixture of reduced and oxidized glutathione. After initiation of disulfide bond formation associated with the folding process of reduced human lysozyme, molecules have been trapped in a stable form with iodoacetic acid (preserving disulfide bonds) at various times of reoxidation. Each population of molecules trapped in this way was then analyzed by acrylamide gel electrophoresis which separates intermediates on the basis of the number of disulfide bonds they contain and the mean volume of the polypeptide chain. Moreover, the rate of reoxidation of the regeneration mixture was monitored by changes in enzymatic activity, fluorescence quantum yield, and global sulfhydryl group titer. Enzymatic activity was observed to appear after an induction period, and no intermediate, except the fully regenerated species, is active. The first two disulfide bonds reoxidize rapidly, and very few intermediates containing one or two disulfide bonds could be trapped. On the other hand, the intermediates containing three and four disulfide bonds are more predominant, and their formation proceeds more slowly. A folding pathway is suggested, based on the kinetic studies of appearance and disappearance of the various observed intermediates. When these results are compared with those obtained for hen egg white lysozyme and with those found in literature, it can be concluded that the reduced human protein recovers its native conformation more progressively and with more difficulty than the hen egg white protein. This difference might be explained by a greater organization and a greater hydrophobicity in the human molecule.