The Size and Number of Polypeptide Chains in Human Serum Low Density Lipoprotein

Abstract Human low density lipoprotein (LDL) was prepared by successive ultracentrifugations of serum at different densities. Apo-LDL, prepared by delipidation of LDL by successive extractions with ethanol-ether and ether, was reduced and carboxymethylated in 7 m guanidine hydrochloride. The molecular weight of apo-LDL was determined by three methods: sodium dodecyl sulfate-polyacrylamide gel electrophoresis, equilibrium ultracentrifugation in 7.6 m guanidine hydrochloride, and Sepharose 4B gel filtration in 6 m guanidine hydrochloride. The molecular weights, as determined by the first two methods, were 255,000 ± 10% and 250,000 ± 5%, respectively. As the polypeptide eluted close to the void volume on the gel filtration column only a range of 275,000 ± 15,000 can be established for its molecular weight. However, significantly, these chromatographic experiments indicated the absence of smaller components. The data provide evidence that only two polypeptide chains, of equal size, can be found in LDL (native molecular weight 2.7 x 106, 20% protein by weight), and that these chains are not aggregates of smaller polypeptide chains. These results are not consistent with the model for LDL proposed by Pollard, H., Scanu, A. M., and Taylor, E. W. ((1969) Proc. Nat. Acad. Sci. U.S.A. 64, 304).