The disulfide folding pathway of bovine pancreatic trypsin inhibitor (BPTI) is characterized by the predominance of folding intermediates with native-like structures. Our laboratory has recently analyzed the folding pathway(s) of four 3-disulfide-containing proteins, including hirudin, potato carboxypeptidase inhibitor, epidermal growth factor, and tick anticoagulant peptide. Their folding mechanism(s) differ from that of BPTI by 1) a higher degree of heterogeneity of 1-and 2-disulfide intermediates and 2) the presence of 3-disulfide scrambled isomers as folding intermediates. To search for the underlying causes of these diversities, we conducted kinetic analyses of the reductive unfolding of these five proteins. The experiment of reductive unfolding was designed to evaluate the relative stability and interdependence of disulfide bonds in the native protein. It is demonstrated here that among these five proteins, there exists a striking correlation between the mechanism(s) of reductive unfolding and that of oxidative folding. Those proteins with their native disulfide bonds reduced in a collective and simultaneous manner exhibit both a high degree of heterogeneity of folding intermediates and the accumulation of scrambled isomers along the folding pathway. A sequential reduction of the native disulfide bonds is associated with the presence of predominant intermediates with native-like structures. -Cys 38 ) (1). Unfolded and fully reduced BPTI refolds spontaneously to form the native structure under selected redox conditions. The mechanism of BPTI unfolding and refolding has been a subject of intensive investigation and represents one of the best and most extensively characterized models (2-7). The folding pathway of BPTI was elucidated by trapping and structural characterization of disulfide bond intermediates. The original model of Creighton (2-5) identified seven predominant intermediates, two 1-disulfide species and five 2-disulfide species, with 75% of the disulfide bonds being native. The native disulfide of BPTI (Cys 30 -Cys 51 ) was found to be a major component of both 1-and 2-disulfide intermediates. In the revised model proposed by Weissman and Kim (6, 7), five well populated intermediates, two 1-disulfide and three 2-disulfide species, were described, and all of them were shown to contain only native disulfide bonds. Another important feature of the BPTI folding pathway is the kinetic role of a 2-disulfide intermediates that contains two of disulfide bonds of the native BPTI (Cys 30 -Cys 51 , Cys 5 -Cys 55 ) (N SH SH ) (3,5,6,8). N SH SH is the immediate precursor of the native BPTI. Formation of the third native disulfide, Cys14 -Cys38, completes the folding and accounts for the final step of the BPTI folding pathway. Prevalence of the native disulfide bonds along the folding pathway has major implications. It implies that non-covalent specific interactions that stabilize the native BPTI and local structures of BPTI play a crucial role in guiding the folding in its early stages and hence dictate the ...