The low-density-lipoprotein (LDL) receptor is a cell-surface protein that plays an important part in the metabolism of cholesterol by mediating the uptake of LDL from plasma into cells. Although LDL particles bind to the LDL receptor through their apolipoprotein B (apo B) and apolipoprotein E (apo E) moieties, other apo E-containing particles, like chylomicron remnants, are not dependent on the LDL receptor for uptake into cells. Chylomicrons formed in the intestinal mucosa during the absorption of the products of digestion, are processed by the peripheral circulation by lipoprotein lipase, which catalyses the breakdown of triglycerides in chylomicrons to free fatty acids and glycerol. The resulting chylomicron remnants, which are cholesterol-rich lipoproteins, are subsequently taken up in the liver. A second distinct protein that binds to apo E-containing lipoproteins, but not to LDL, has been proposed to be the receptor mediating the clearance of chylomicron remnants from the plasma. This protein has a relative molecular mass (Mr) of 56,000 (56K). More recent studies have failed, however, to establish whether this protein is a cell-surface receptor. Here we describe crosslinking experiments in which apo E liposomes were found to bind specifically to the cell surface of hepG2 cells and to human liver membranes. The size and immunological cross-reactivity of the protein to which the liposomes bound was indistinguishable from that of the recently cloned and sequenced LDL-receptor-related protein, LRP. We therefore conclude that the LRP might function as an apo E receptor.

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The apolipoprotein composition of purified human Lp(a) lipoprotein was investigated by SDS-polyacrylamide gel electrophoresis and immunochemically. The lipoprotein contains two different polypeptides. One is identical by its app. AJ& of -250000 and immunologically with apohpoprotein B of LDL (B-100). The other polypeptide has a higher app. M, (-3SOOUO) and stains strongly with the periodate-Schif~s reagent. This high-M, glycoprotein contains the specific Lp(a) immunoreactivity but does not react with antibodies against apo B. Apo B and Lp(a)-protein seem to be linked by disulfide bonds in the native lipoprotein. The unreduced detergent delipidized protein moiety from Lp(a) lipoprotein shows a single band of M, -700000 in SDS-polyacrylamide gel electrophoresis and the immunoprecipitates formed against anti-Lp(a) and anti-apo B by the unreduced protein show a reaction of immunological identity. Lp(a) lipoprotein Lp(a) antigen A therosclerosis

Based on our patient material, preoperative localization of insulinoma was correct with sonography in 13 (61.9%) of 21 patients, with computed tomography in 3 (21.4%) of 14 patients, with computed tomography with bolus injection of contrast medium in 11 (73.3%) of 15 patients, with angiography in 20 (66.6%) of 30 patients, and with percutaneous transhepatic portal vein catheterization with selective measurement of insulin in 10 (76.9%) of 13 patients. Intraoperatively, 40 (95.2%) of 42 insulinomas were palpable and 12 of 16 insulinomas were identified during intraoperative sonography. Although 95.2% of the insulinomas can be palpated, we would support additional diagnostic localization since it may improve the reliability of palpation.