The Microheterogeneity of Plasma Albumins. Ii. Preparation and Solubility Properties of Subfractions.

In the first paper of this series (1) a model for microheterogeneity of plasma albumins was presented together with some critical evidence in support of its adoption. According to this model plasma albumin consists of a very large number of similar molecular species. The N-F transition of the protein as observed by electrophoresis, solubility, optical rotatory dispersion, and other methods is actually the resultant of a heterogeneity distribution function1 superimposed on the pH dependence of individual transitions. Each component species undergoes an immeasurably fast, highly cooperative isomerization reaction at a slightly different characteristic pH. At a specific pH within the over-all N-F transition region most of the molecules exist virtually 100% of the time in either the N or the F form. ,4 minor fraction of the molecules is in effective dynamic equilibrium between N and F forms. This last category comprises species whose individual N-F transitions occur at or close to the prevailing pH of the solution. Experiments based on the difference in solubility of N and F forms in the presence of high salt concentrations were cited (1) in substantiation of the microheterogeneity model. As a logical application of the solubility property it should be possible to fractionate the population into narrower subpopulations by removing precipitate at various values of pH within the range of the N-F transition. The subpopulations thus obtained should exhibit N-F transitions different from each other and from the parent population. Only a subpopulation precipitated symmetrically about the pH midpoint of a symmetrical distribution would be expected to display an N-F transition with a midpoint that coincides with that of the whole population. Moreover, the observed transition of any subfraction should cover a narrower pH range than that of the parent albumin. Herein we discuss the preparation of plasma albumin subfractions, their solubility properties, and related problems. A subsequent paper of this series will concern other physicochemical studies of such subfractions.2 Some electrophoretic results have already been reported (2).