title: Biphasic denaturation of human placental alkaline phosphatase in guanidinium chloride.
abstract: Human placental alkaline phosphatase is a membrane-anchored dimeric protein. Unfolding of the enzyme by guanidinium chloride (GdmCl) caused a decrease of the fluorescence intensity and a large red-shifting of the protein fluorescence maximum wavelength from 332 to 346 nm. The fluorescence changes were completely reversible upon dilution. GdmCl induced a clear biphasic fluorescence spectrum change, suggesting that a three-state unfolding mechanism with an intermediate state was involved in the denaturation process. The half unfolding GdmCl concentrations, [GdmCl]0.5, corresponding to the two phases were 1.45 M and 2.50 M, respectively. NaCl did not cause the same effect as GdmCl, indicating that the GdmCl-induced biphasic denaturation is not a salt effect. The decrease in fluorescence intensity was monophasic, corresponding to the first phase of the denaturation process with [GdmCl]0.5 = 1.37 M and reached a minimum at 1.5 M GdmCl, where the enzyme remained completely active. The enzymatic activity lost started at 2.0 M GdmCl and was monophasic but coincided with the second-phase denaturation with [GdmCl]0.5 = 2.46 M. Inorganic phosphate provides substantial protection of the enzyme against GdmCl inactivation. Determining the molecular weight by sucrose-density gradient ultracentrifugation revealed that the enzyme gradually dissociates in both phases. Complete dissociation occurred at [GdmCl] > 3 M. The dissociated monomers reassociated to dimers after dilution of the GdmCl concentration. Refolding kinetics for the first-phase denaturation is first-order but not second-order. The biphasic phenomenon thereby was a mixed dissociation-denaturation process. A completely folded monomer never existed during the GdmCl denaturation. The biphasic denaturation curve thereby clearly demonstrates an enzymatically fully active intermediate state, which could represent an active-site structure intact and other structure domains partially melted intermediate state.
authors.affiliation: Graduate Institute of Life Sciences, National Defense Medical Center, Taipei, Taiwan, Republic of China.
authors.forename: H C
authors.initials: HC
authors.lastname: Hung
authors.forename: G G
authors.initials: GG
authors.lastname: Chang
mesh_terms: D000469:Alkaline Phosphatase / Q000737:chemistry*; D002942:Circular Dichroism; D019791:Guanidine / Q000737:chemistry*; D006801:Humans; D010710:Phosphates; D010920:Placenta / Q000201:enzymology*; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D013050:Spectrometry, Fluorescence
chemical_list: D010710:Phosphates; D000469:Alkaline Phosphatase; D019791:Guanidine