Reversed-phase high-performance liquid chromatography of red blood cell membranes

Purification of membrane proteins

As stated at the beginning of this chapter, it is not possible to present a single step-by-step protocol for the purification of all membrane proteins. We have discussed many of the individual techniques employed to purify membrane proteins and the problems associated with their application. A successful purification protocol will require the use of a combination of these techniques. It should be clear from the examples given, however, that affinity chromatography is by far the most useful technique available. The greatest single obstacle to performing a successful purification is the ability to maintain solubilized proteins in fully dispersed monomeric micelles without inactivating the protein. The best combination of techniques and the optimal conditions for their use can be determined only by trial and error. This process should become easier as a greater number of examples become available and as greater use is made of HPLC and FPLC techniques.

High-performance liquid chromatographic separation of membrane proteins isolated from erythrocyte ghosts

Membrane proteins extracted from erythrocyte ghosts with sodium dodecyl sulphate (SDS), 3-(3-cholamidopropyl)-dimethylamminopropane sulfonate (CHAPS) or octylglucoside have been analyzed in various reversed-phase high-performance liquid chromatographic systems. Only SDS was able to solubilize considerable amounts of membrane proteins with mol.wt. greater than 15,000 daltons, but these membrane proteins were recovered in poor yield from a silica-based C4 column eluted with an acetonitrile gradient in trifluoroacetic acid (TFA). A resin-based phenyl column eluted with a similar TFA-acetonitrile gradient was found to be a better choice with respect to the recovery of membrane proteins with mol.wt. greater than 15,000 daltons, and when this column was eluted with an acetic acid gradient with increasing amounts of acetonitrile, erythrocyte ghost membrane proteins solubilized in SDS (mol.wt. 10,000-200,000 daltons) were separated in six major and several minor components with satisfactory recovery.

Chromatography of complex protein mixtures

This review has shown that a variety of chromatographic techniques are available for fractionating proteins. Fortunately, high-quality columns of every type described in this review are commercially available. Most water-soluble proteins may be eluted from size-exclusion, hydrophobic-interaction, ion-exchange, metal chelate, and bioaffinity columns with ease. When this is the case, high recovery and retention of biological activity are the norm. The exception is reversed-phase chromatography where the organic solvents and acids used in polypeptide elution denature many proteins. When problems do occur, they are generally the result of unique structural features of the protein. Very hydrophobic proteins have presented the biggest problem in that they are difficult to solubilize, particularly with retention of biological activity. It has been found that zwitterionic and non-ionic detergents are the most suitable solubilizing agents, but urea has also been used in cases where hydrophobic interacts are not as strong. Unfortunately, there is still an element of trial-and-error in selecting the most suitable solubilizing agent. Heterogeneous glycosylation of proteins also presents a problem. Both neutral and charged monosaccharides can be incorporated into proteins through multiple steps at several sites. Thus, there is the potential in a sample for a large number of glycoprotein species which have the same polypeptide backbone and differing amounts of oligosaccharide. A problem arises when size-exclusion, ion-exchange, hydrophobic-interaction, reversed-phase and bioaffinity systems begin to discriminate between these very similar glycoprotein species. Chromatographic peaks can become very broad, due to incomplete fractionation, and the polypeptide chain of interest can be associated with multiple peaks. The separation of glycoproteins requires much more study before logical procedures can be suggested for column selection and operation. Aggregated species are another class of proteins which present occasional problems. Multimeric proteins are adsorbed to sorbents by a series of forces, among which are hydrogen bonding, hydrophobic interactions, and electrostatic forces. These forces are also responsible for the maintenance of quaternary structure in proteins. When the same forces dominate both retention of protein structure and adsorption at the sorbent surface, the quaternary structure of the protein can be disrupted during elution. Very basic proteins also present a problem in some cases. Columns with residual negative charges, such as a silica-based reversed-phase column, adsorb anionic species so strongly that they are difficult to elute.(ABSTRACT TRUNCATED AT 400 WORDS)

High-performance liquid chromatography of membrane proteins

Various modes of high-performance liquid chromatography, gel filtration, ion-exchange chromatography, hydrophobic interaction chromatography, reversed-phase chromatography and metal chelate affinity chromatography, were investigated for the separation of membrane proteins. All were found applicable to membrane proteins, although the usefulness of each mode differed. For satisfactory results it was important to select appropriate elution conditions. The type and concentration of detergent was of special importance. The effects of other conditions, flow-rate, gradient steepness, type of buffer and salt, eluent pH, etc., were similar to those observed for soluble proteins.