Drop dialysis: time course of salt and protein exchange.

By drop dialysis with membrane filters of 25 or 50 nm average pore size, salt concentrations are reduced to 15% within 25 min. During this time only 10% of ribonuclease with a Mr 13,500 will diffuse in and through the membrane. However, in the presence of 1 M NaCl about 25% of the enzyme is lost. The difference in the rate of salt removal and enzyme loss is caused by the difference in diffusion constants. Therefore with enzymes of higher molecular weights, less protein will be lost, as is shown with beta-galactose dehydrogenase. This enzyme with Mr 64,000 is lost at a lower rate than ribonuclease. The net charge of a protein apparently does not influence the rate with which it diffuses through the membrane. The time course of salt and protein exchange was studied to provide data for estimating the optimal conditions for the required reduction in salt concentration. To prepare small protein samples for electrophoresis or other analytical methods, which require low salt concentrations or a buffer change, drop dialysis is a fast and effective method with tolerable loss of protein.