Sodium dodecyl sulphate-protein complexes : Changes in size or shape below the critical micelle concentration, as monitored by high-performance agarose gel chromatography

Abstract

We have determined the sodium dodecyl sulphate (SDS) concentration needed to complete the formation of SDS-protein complexes. A superose-6 column was equilibrated with SDS for 7 h. A sample of a native protein or an SDS-protein complex was applied, and the elution volume, V e, was determined. Then the SDS concentration, C SDS, was changed, etc., i.e., V e was determined as a function of C SDS. The critical micelle concentration of SDS (cmc SDS) was 1.8 m M in the eluent (ionic strength 0.10 M). Native bovine carbonic anhydrase (BCA) formed an SDS complex above 0.2 m M SDS. As C SDS was increased, V e decreased gradually in two main transitions, (TI) at 0.2–1.0 m M and (TII) at 1.2–2.0 m M SDS. These concentrations are corrected for a lag in the column equilibration with SDS. SDS-BCA, pre-equilibrated at 1.6 m M SDS, showed transitions similar to those observed with native BCA, except that transition TII included a minor transition at 2.0–2.2 m M SDS. The SDS complexes of reduced and carboxamidomethylated bovine serum albumin, of N-5′-phosphoribosylanthranilate isomerase-indole-3-glycerol-phosphate synthase from Escherichia coli (PRAI-IGPS) and of two tryptic fragments of this enzyme behaved similarly. For SDS-PRAI-IGPS the major part of transition TII was completed at 1.6–1.7 m M SDS, as shown by analyses after 20-h column equilibrations with increasing as well as decreasing C SDS. The SDS complex of an integral membrane protein, the glucose transporter from human red cells, was smaller or less elongated than the SDS complexes of water-soluble proteins of the same polypeptide length. The formation of all five SDS-protein complexes investigated was practically completed at cmc SDS.