Question

Examine the effects of the detergent sodium dodecyl sulfate (SDS) on the binding activity of serium albumin (this detergent is a strong protein denaturant).

Answer 1

The study of the binding of large amounts of detergent to proteins should furnish important insights into the more complicated and less easily understood problems of lipid-protein interactions. The lipoproteins of serum have been available in large amounts for years, yet the location of the various types of lipids in relation to the protein side chains and to each other is simply not known. Membrane structure is likewise not understood in detail. It would seem simpler to approach the understanding of these complex structures by studying the simpler interaction of two
components-the hydrophobic negatively charged dodecyl sulfate (the lipid) and various proteins. Second, in attempting to dissociate a few proteins resistant to 6 M guanidine hydrochloride, it was thought possible that sodium dodecyl sulfate, perhaps in combination with urea, might dissociate these more efficiently. This has been found true for mitochondrial structural protein.The binding of sodium dodecyl sulfate to proteins has been studied since the mid-1940’s.

It was found by dialysis equilibrium that proteins (and their polypeptide chains) bind as much as 1.4 g of SDS per g of protein. I have confirmed these large binding capacities by measurements of the residual micelle in the presence of proteins (disulfides intact) using ultracentrifugal photographs of the SDS micellar area when it had separated from the faster sedimenting protein. By raising the ionic strength and in the presence of much higher concentrations of SDS than previously used, the amount bound can be raised to around 2 g of SDS per g of protein.

In retaining the proteins in their native conformation before addition of SDS, it was found that they each behaved somewhat differently, exhibiting different time dependences, somewhat different amounts of SDS bound at equilibrium, and different kinetics of subunit formation-if these are present. Finally, some proteins do not bind measurable amounts of SDS, and can retain full activity in its presence. The three resistant proteins found thus far are pepsin, papain, and glucose oxidase, all of which are known to be very stable under some (but not all) types of denaturing conditions.