A method of measuring protein dry weight in solutions of unknown salt concentration and an application to lipoproteins

Abstract

The dialysis bag dry weight method was developed for the measurement of dry weights of lipoproteins subfractionated on density gradients where total recovery of lipoprotein in the gradient was to be determined. Use of the conventional method for dry weight determination was precluded because of inconsistent concentration changes which would occur in the dialysis step due to differences in both the lipoprotein and salt concentration among gradient fractions. The method described consists of transfer of measured undialyzed samples into previously weighed bags followed by dialysis against water, lyophilization of the protein-bag combination and calculation of the protein dry weight as the difference between the bag weight and the total weight.Since the method described incorporates dialysis in the assay, it is capable of giving an accurate protein dry weight measure of a non-predialyzed sample, whereas the conventional dry weight method gives an accurate value only of a previously dialyzed sample. The increase in the standard deviation of the overall dialysis bag method was shown to be less than double that of the conventional method for a sample of known salt concentration and there is no distinguishable difference in the central values obtained by the two methods.The particular usefulness of this method for lipoprotein solutions was presented.