Molecular complexes in the isolation and characterization of plasma lipoproteins.

T h e formation of insoluble complexes between sulfated polysaccharides and certain plasma lipoproteins was independently described by several investigators in 1955 (1, 2, 3). These complexes are formed a t neutral pH and in the absence of organic solvents, and are readily dissociated by increasing ionic strength or by the addition of chelating agents which remove certain metal ions that appear to be necessary for the formation of the complex. Precipitation methods for the isolation and purification of plasma lipoproteins and turbidimetric procedures for their quantitative estimation based on this phenomenon have been developed. These methods satisfy many of the criteria proposed by Lindgren and Nichols (4) for the evaluation of lipoprotein isolation techniques. The mild conditions required for complex formation and dissociation, as well as the convenience and reproducibility of the methods, make them particularly attractive for the investigation of plasma lipoproteins. Comparable isolation procedures involving the formation of complexes between lipoproteins and the neutral polymer polyvinylpyrrolidone have also been described recently (5). In this review the formation of complexes between lipoproteins and macromolecules will be discussed and methods employing complex formation for the isolation and quantitation of plasma lipoproteins evaluated. Since the turbidimetric procedures are comparable in some ways to the older thymol (6) and phenol (7) turbidity tests for the estimation of y-globulins and lipoproteins, a discussion of these is also included in this review. Fractionation procedures such as the Cohn cold-ethanol Methods 6 and 9 and Method 10, although frequently employed in lipoprotein studies,