| Abstract: | Lipoproteins of density 1.063--1.21 g/ml were isolated from the plasma of three sisters of Irish origin with familial LCAT deficiency. Fractionation of the lipoproteins on the basis of particle size by chromatography on Sephacryl S-300 permitted partial separation of two major and at least three other minor components which differed in their lipid:protein ratio and their apolipoprotein content. One of the major components was a small spherical lipoprotein whose sole apolipoprotein was apoA-I; the second major component contained predominantly apoA-I, together with apoE, and in addition, an apolipoprotein of molecular weight 46,000 that was not cleaved by reduction of disulfide bonds, and which was identified as apoA-IV. This apoprotein has not previously been detected in the lipoproteins of LCAT-deficient patients. A second apoE-containing lipoprotein, which contained apoA-I and apoE in a ratio of approximately 2:1, was also present as a minor component, together with two or more minor components whose apoproteins were comprised of apoA-I and apoC. The apoE-containing lipoproteins competed efficiently with 125I-labeled LDL for binding to high affinity LDL-receptor sites on the surface of cultured human skin fibroblasts. The ability to bind to the LDL-receptor was directly proportional to the apoE content of the lipoproteins, even when other apoproteins, with the exception of apoB, were present in relatively large proportions. ApoE-containing 125I-labeled lipoproteins from an LCAT-deficient subject were also taken up and degraded by the cultured cells. |
