Dissociation-Association Behavior of Arachin between Dimeric and Monomeric Forms

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Abstract
Two molecular species of arachin (I and II), which formed different dissociation-association systems between dimer and monomer, were purified. Arachin I existed as monomer (mol. wt., 180,000) in 0.01 m sodium phosphate buffer, pH 7.9, but it associated reversibly to dimer (350,000) in 0.3 m sodium phosphate buffer, pH 7.9. While, arachin II existed as dimer (350,000) under both conditions. By lowering pH to 6.8 at ionic strength 0.03, arachin I monomer was reversibly associated to its dimer, whereas arachin II dimer was partially dissociated to its monomer. The monomer once formed from arachin II by lowering pH to 6.8 underwent the reversible association, similarly to arachin I. Subunit and amino acid compositions did not make a clear distinction between arachin I and II. These facts suggest that there are two types of arachin dimers differing in dissociation behavior to the respective monomers and that the difference in their dissociation behavior may be due to the slight difference in conformation of their monomers.

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