The effect of sodium dodecyl sulfate (SDS) on the activity and conformation of soybean peroxidase (SBP) was studied by the measurement of activity, far-UV circular dichroism spectra, electronic absorption spectra, and fluorescence spectra at different pH values and 30 ℃. In the pH 2.6 and 4.2 systems, the interaction of SDS at a submicellar concentration with SBP and then with His169 of SBP due to the electrostatic attraction weakens the binding between imidazole of His169 and Fe(III) of heme. As a result of this change, the influence of Fe(III) of heme on porphrin ring is increased leading to the blue shift of Soret band. In the meantime, the secondary structure of SBP changes slightly and the activity is lost irreversibly. After the activity of SBP is lost, the α-helix content of SBP is decreased but that of β-strand increased gradually along with the increment of the SDS concentration. In pH 5.2 systems, SDS can not get into the SBP molecule to bind to the His169 since it has the same kind of charge with SBP. When the SDS concentration is lower than its cmc, it has no obvious effect on the secondary structure of SBP, but only has little effect on the tertiary structure. When the SDS concentration is increased above its cmc, the electrostatic repulsive force between the micelles of SDS and SBP is increased evidently so as to restrict the movements of the vinyl substituents in heme of SBP molecules. Therefore the conjugation between the vinyl substituents and the porphrin ring is enforced, which leads to the red shift of Soret band of SBP and change of the tertiary structure. This change is reversible since the SBP activity is recoverable. In conclusion, the precondition of SBP denaturalization is the electrostatic interaction between SDS and SBP, and His169 binding to the heme is the key element of SBP activity.

Key words: sodium dodecyl sulfate, soybean peroxidase, activity, conformation