SDS (sodium dodecyl sulfate) effect on the autoxidation of the Glossoscolex paulistus giant extracellular hemoglobin:: Kinetic studies at pH 7.0 and 9.0

The effect of the anionic surfactant sodium dodecyl sulfate (SDS) on the autoxidation process of the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) is addressed in the present work. The complex oligomeric assembly of hemoglobin subunits may influence the autoxidation rate and the exponential decay behavior. Kinetic studies were developed using UV-vis measurements at 415 nm. These spectroscopic measurements are analyzed at two pH values, 7.0 and 9.0, where the hemoglobin presents different oligomeric assembly. At pH 7.0 a high stability of the native form of the oxy-hemoglobin is observed, while at pH 9.0 an intense dissociation of the oligomer is promoted by alkalization. This difference is evident by comparison of the rate constants in the absence of surfactant: at pH 7.0 the kinetics presents a mono-exponential behavior with a rate constant of 0.27 x 10(-4) s(-1) while at pH 9.0 a bi-exponential behavior was observed with rate constant increase to 7 x 10(-4) s(-1) (fast process) and I X 10(-4) s(-1) (slow process). In the autoxidation induced by SDS two factors affect significantly the process rate, namely, the oligomeric arrangement of the hemoglobin and the strength of the interaction between SDS and HbGp. At pH 7.0, for SDS concentrations up to 0.3 mm, a mono-exponential behavior was observed, showing rate constants around 0.4 x 10(-4) s(-1), which suggest that the hemoglobin still maintains the more compact structure observed at this pH for the native protein. In the SDS concentration range 0.75-1.0 mm, the mono-exponential process changes into a bi-exponential behavior with rate constants varying from 48 x 10(-4) Up to 99 X 10(-4) s(-1) for the fast process and from 1.7 x 10(-4) up to 3.7 x 10(-4) s(-1) for the slow process, suggesting hemoglobin dissociation. At pH 9.0, a bi-exponential decay is observed for all studied SDS concentration range, presenting rate constants from 11.0 X 10(-4) Up to 179 x 10(-4) s(-1) for the fast process and from 1.0 X 10(-4) Up to 8 X 10(4) s(-1) for the slow process probably due to hemoglobin dissociation, which is already present in the absence of surfactant. At pH 7.0, the highly packed native protein structure should inhibit the autoxidation process, but the SDS/HbGp interaction is more intense as compared to pH 9.0, due to the acid pI value, promoting oligomeric dissociation. So, the autoxidation process is regulated at pH 7.0 by the interaction with SDS, which triggers oligomeric dissociation and increase of autoxidation rate. At pH 9.0, the autoxidation process should be very fast, probably due to the oligomeric dissociation, which is already present in the absence of surfactant. At alkaline pH, the interaction with SDS seems be weaker than at pH 7.0. This behavior at pH 7.0 can be observed through the higher autoxidation rate for the faster chains and it is associated to the acid pI of the giant extracellular hemoglobins. (c) 2006 Elsevier B.V. All rights reserved.