Purification of thermamylase in multicompartment electrolyzers with isoelectric membranes: The problem of protein solubility

The main alpha-amylase from Bacillus licheniformis (called thermamylase because of its resistance to high temperatures, 90 degrees C) has been subjected to purification by isoelectric focusing in multicompartment electrolyzers with isoelectric membranes. The enzyme tended to precipitate, producing severe smears in proximity of its pI value (7.18). Solubility could not be ameliorated by any of the known means typically adopted in isoelectric focusing and compatible with enzyme activity, such as addition of neutral and zwitterionic surfactants (e.g., Nonidet, 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate, up to 2%), mixed hydro-organic solvents (glycerol, ethylene glycol, propylene glycol) and addition of zwitterions unable to form micelles, such as taurine. However, addition of sugars, notably saccharose, sorbitol, and, to a lesser extent, sorbose, greatly improved protein solubility in the pI proximity. The improvement was dramatic if these sugars were admired with 0.2 M taurine; Additionally, the increment of solubility (which occurred when reaching a level of 40% of the different sugars) was accompanied by a large pI shift, typically reducing the pI value by as much as 0.4 pH units (e.g., from a pI of 7.18 in the absence of additives to a pI of 6.80 in presence of a mixture of 40% sucrose and 0.2 M taurine, the best solubilizer in all the series investigated). This apparent pI shift was not due to a change of pH gradient caused by the presence of additives, since pH measurements in the absence as well as presence of additives gave identical results. The results are explained by the theory of Timasheff and Arakawa on stabilization of protein structure by solvents: sugars (at ca. 1 M concentration) and zwitterions such as taurine belong to class I stabilizers, characterized by negative binding to proteins and by increasing the surface tension of water. As a result, the protein is in a state of ''superhydration'' which might prevent binding to Immobilines in the gel matrix and might alter some pKs on the protein surface. In solutions of 40% saccharose and 0.2 M taurine, thermamylase could be successfully purified to a single isoelectric and isoionic band in the multicompartment electrolyzer.