Sodium dodecyl sulphate-protein complexes. Changes in size or shape below the critical micelle concentration, as...

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, Ve, was determined. Then the SDS concentration, CSDS, was changed, etc., i.e., Ve was determined as a function of CSDS. The critical micelle concentration of SDS (cmcSDS) was 1.8 mM in the eluent (ionic strength 0.10 M). Native bovine carbonic anhydrase (BCA) formed an SDS complex above 0.2 mM SDS. As CSDS was increased, Ve decreased gradually in two main transitions, (TI) at 0.2-1.0 mM and (TII) at 1.2-2.0 mM SDS. These concentrations are corrected for a lag in the column equilibration with SDS. SDS-BCA, pre-equilibrated at 1.6 mM SDS, showed transitions similar to those observed with native BCA, except that transition TII included a minor transition at 2.0-2.2 mM 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 mM SDS, as shown by analyses after 20-h column equilibrations with increasing as well as decreasing CSDS. 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 cmcSDS.