Electrophoretic analysis of horse tissue ferritins at different pH values

Low-tech electrophoresis, small but beautiful, and effective: electrophoretic titration curves of proteins

Migration across a stationary pH gradient results in the electrophoretic titration of a protein's dissociable groups. From the resulting curves, some properties of the protein may be derived, including overall amino acid composition and type of mutation between polymorphic variants, as well as range of stability or, for enzymes, of catalytic activity. Analysis with this technique is a stringent purity criterion; other applications allow the study of interacting systems and the planning of chromatographic fractionations based on differences in surface charge.

Denaturation and self-association of apolipoprotein A-I investigated by electrophoretic techniques

Purified human apolipoprotein A-I (apoA-I), when run across a transverse urea gradient at alkaline pH, gives a complex pattern characterized by a number of parallel sigmoidal curves, in which the transition between high- and low-mobility forms, i.e. from folded to unfolded structure, occurs between 1.1 and 3.2 M urea. Size differences appear to be the major cause of this isomerism. When migrated across a wide pH range in the presence of varying amounts of urea to display its titration curve, apoA-I is resolved into two pairs of bands, running parallel in the neutral to basic pH region while merging at acidic pH; such a finding does not correlate with a differential exposure of His residues, as shown by diethyl pyrocarbonate titration. Ferguson plot analysis, confirmed by cross-linking experiments, demonstrates a gradual shift from higher to lower mass aggregates as the urea concentration is raised; the monomeric form undergoes denaturation by swelling to an approximately 50% larger hydrodynamic volume than in its native state. At alkaline pH, where apoA-I exists as aggregated species, disaggregation and unfolding appear to happen at once, the larger aggregates being less stable than the smaller ones. At acidic pH, apoA-I does not form aggregates and has little secondary structure; unfolding is then a progressive rather than a cooperative process.

Isoelectric focusing as a tool for the investigation of post-translational processing and chemical modifications of proteins

It has been demonstrated that good agreement may be observed between computed and experimental isoelectric point (pI) values when proteins of known sequence are focused under denaturing conditions on immobilized pH gradient IPG slabs, at least in the pH range 4-7.5. Hence, discrepancies between expected and found in this experimental set-up may be reliably ascribed to some kind of post-transcriptional processing, or chemical modification, having taken place in the sample. This evaluation is made easier when the comparison is set between the pI of a parent molecule and that (or those) of one to several of its derivatives as resolved in a single experiment (for instance, as a spot row in two-dimensional maps); no previous knowledge is required in these cases about the amino acid composition of the primary structure. The effects on protein surface charge are discussed in this review mainly for two biologically relevant processes, glycosylation and phosphorylation. Then, the pI shifts are analysed for some protein modifications that may occur naturally but can also be artefactually elicited, such as NH2 terminus blocking, deamidation and thiol redox reactions. Finally, carboxymethylation and carbamylation are used to exemplify chemical treatments often applied in connection with electrophoretic techniques and involving charged residues. Procedures to be applied in order to verify whether a given modification has occurred, and often relying on the focusing of a treated specimen, are detailed in each section. Numerical examples on model proteins are also discussed. As an important field of application of the above concepts may be genetic engineering, an exhaustive bibliographic list dealing with pI evaluation and structural assessment on recombinant proteins is included.

Selective crystallization of horse isoferritins

Various precipitating agents were examined in order to crystallize horse heart and spleen ferritins. Cadmium sulfate induced the crystallization of the spleen ferritin, while 2-methyl-2,4-pentanediol and poly(ethylene glycol) only induced that of the heart ferritin. Isoelectric focusing analysis showed that the crystals grown from cadmium sulfate contained only the more acidic isoferritins, and those grown from methyl pentanediol only the less acidic isoferritins. Heart ferritin crystallizes in a cubic space group, as previously reported for spleen ferritin crystals grown from cadmium sulfate.