Mechanism of lipid-protein interaction in human plasma high density apolipoprotein A-II

[[abstract]]Apolipoprotein A-II (apoA-II) is a dimeric 77-residue apoprotein of human high-density lipoproteins. Previous studies indicate that residues 56--77 in the apoprotein do not bind phospholipid whereas residues 47--77 form a complex with dimyristoylphosphatidylcholine (DMPC). To further delineate the lipid-binding region between residues 47 and 77, we have prepared synthetic fragments of apoA-II corresponding to residues 54--77, 52--77, and 50--77 and have tested each fragment for its ability to interact with vesicles of DMPC. The interaction of the fragments was determined by changes in secondary structure as measured by circular dichroism and by isolation of peptide--DMPC complexes by ultracentrifugation in density gradients of KBr. By these criteria, only fragment 50--77 binds DMPC; there is an increase in alpha helicity from 17% to 41% when the fragment associates with lipid. Since the 56--77 fragment does not associate with phospholipid, we propose that the addition of residues Thr-Pro-Leu-Ile-Lys-Lys (corresponding to residues 50--55) to the 56--77 fragment gives the peptide the necessary sequence information for lipid binding. To further identify the important amino acid residues in the region of 50--55, we have substituted Leu-Ile with Ala-Ala. This substitution totally abolishes the lipid-binding capacity of the 50--77 fragment. On the other hand, substitution of Lys-Lys with Ser-Ser does not alter the lipid-binding capacity of the peptide. We conclude that residues 50--55 are important in lipid binding and that the hydrophobic center formed by Leu-Ile plays an important role

Physical, chemical, and immunochemical studies of apolipoprotein A-I from pigeon plasma high density lipoproteins

[[abstract]]Pigeon plasma high density lipoproteins (HDL) were isolated by ultracentrifugation between the densities of 1.063 and 1.21 g/ml. Gel filtration of delipidated HDL in 5 M guanidine-HCl on Sephadex G-150 yielded a major fraction which eluted at the same position as human apolipoprotein A-I isolated from HDL. In SDS-gel electrophoresis, the isolated apolipoprotein co-migrated with human apolipoprotein A-I with a molecular weight of approx. 28 000. The amino acid composition was similar to the apolipoprotein A-I isolated from human and hen plasma. The isolated apolipoprotein from pigeon plasma had therefore been designated as apolipoprotein A-I. As judged by circular dichroism (CD), the apolipoprotein A-I displayed a maximum mean residue ellipticity of approx. -3 000 at 222 nm while at concentrations greater than 0.2 mg/ml. Calculations of alpha-helicial content gave values of 85%. Lowering the concentration of apolipoprotein A-I was found to concomitantly decrease the ellipticity (absolute value) suggesting that there was some conformational change when the apolipoprotein A-I concentration varied. The isolated pigeon apolipoprotein A-I was found bound to the phospholipid (dimyristoyl phosphatidylcholine) and there was no significant conformational change upon lipid binding as judged by CD. Under the same experimental conditions, human apolipoprotein A-I exhibited a drastic conformational change by increasing its helicity in the presence of phospholipid. The helical content of human apolipoprotein A-I was increased from 48 to 85%. This finding suggests that the apolipoprotein may not necessarily increase its helical content during lipid binding. Moreover, immunochemical studies showed that rabbit antiserum prepared against pigeon apolipoprotein A-I could partially react with human apolipoprotein A-I determined by quantitative radioimmunoassay

Enhancement of limiting dilution in cloning mouse myeloma-spleen hybridomas by human low density lipoproteins

[[abstract]]The limiting dilution technique is a critical step in the cloning of hybridomas for the preparation of monoclonal antibodies. We have found that culture medium supplemented with human plasma low density lipoproteins (LDL) markedly enhanced the yield of hybridoma clones derived from P3 X 63 Ag or FO mouse myeloma cell lines upon limiting dilution. Such enhancement was dependent on the concentration of LDL employed, being optimal at 1-10 micrograms/ml. At LDL concentrations greater than 20 micrograms/ml, the increase in yield of hybrid clones was not significant. The mechanism by which LDL enhances the yield of hybrid clones was partially elucidated by the demonstration that LDL could increase the DNA synthesis of hybridomas as assessed by [3H]thymidine incorporation. The data suggest that LDL play a role in the proliferation of hybridomas. It also indicates that LDL can be utilized for limiting dilution to increase the yield of desired clones. Since LDL is one of the most abundant lipoprotein fractions (approximately 500 micrograms/ml) in human plasma and the isolation procedure is simple, hybridoma culture medium supplemented with human LDL will prove to be a valuable reagent for investigators currently employing monoclonal antibody technology

Mechanism of lipid-protein interaction in the plasma lipoproteins: identification of a lipid-binding site in apolipoprotein A-II.

[[abstract]]Apolipoprotein A-II (apoA-II) is a dimeric 77-residue apoprotein of human high-density lipoproteins. Previous studies indicate that residues 56--77 in the apoprotein do not bind phospholipid whereas residues 47--77 form a complex with dimyristoylphosphatidylcholine (DMPC). To further delineate the lipid-binding region between residues 47 and 77, we have prepared synthetic fragments of apoA-II corresponding to residues 54--77, 52--77, and 50--77 and have tested each fragment for its ability to interact with vesicles of DMPC. The interaction of the fragments was determined by changes in secondary structure as measured by circular dichroism and by isolation of peptide--DMPC complexes by ultracentrifugation in density gradients of KBr. By these criteria, only fragment 50--77 binds DMPC; there is an increase in alpha helicity from 17% to 41% when the fragment associates with lipid. Since the 56--77 fragment does not associate with phospholipid, we propose that the addition of residues Thr-Pro-Leu-Ile-Lys-Lys (corresponding to residues 50--55) to the 56--77 fragment gives the peptide the necessary sequence information for lipid binding. To further identify the important amino acid residues in the region of 50--55, we have substituted Leu-Ile with Ala-Ala. This substitution totally abolishes the lipid-binding capacity of the 50--77 fragment. On the other hand, substitution of Lys-Lys with Ser-Ser does not alter the lipid-binding capacity of the peptide. We conclude that residues 50--55 are important in lipid binding and that the hydrophobic center formed by Leu-Ile plays an important role

Characterization of a monoclonal antibody specific to the amino terminus of the alpha-chain of human fibrin

[[abstract]]A peptide, Gly-Pro-Arg-Val-Val-Glu, corresponding to the first six residues of the amino terminus of the alpha-chain of human fibrin (desAA-fibrin) was prepared by solid-phase peptide synthesis. The peptide was covalently linked to keyhole-limpet hemocyanin (KLH) and used as an immunogen for preparing monoclonal antibodies. A monoclonal antibody specific to the hexapeptide, but not to KLH or fibrinogen, was produced. The antibody did not bind to thrombin-mediated clots prepared from either plasma or purified fibrinogen. However, immunoreactivity was detected when fibrin (prepared from fibrinogen) was solubilized with 8 M urea. In contrast, a monoclonal antibody specific to the amino terminus (Gly-His-Arg-Pro-Leu-Asp-Lys) of the beta-chain of fibrin recognized the epitope in clots. These results indicate that thrombin cleavage of fibrinogen produces a structural change in the amino terminal domain of the alpha-chain that makes it inaccessible to antibody interaction. In addition, our study suggests that the potential clinical application of monoclonal antibodies to localize fibrin-rich thrombi must take into account the final structure of clots

A colorimetric assay for measuring the lysis of a plasma clot.

[[abstract]]The present study describes a simple, quantitative assay for measuring the lysis of a plasma clot. The principle of the assay is based on the release of Coomassie brilliant blue R-250 dye from the clot. Thirty microliters of freshly prepared Coomassie brilliant blue R-250 (1 mg/ml) was added to 200 microliters of diluted human plasma (1:5). After mixing, 100 microliters of thrombin (2.5 NIH units/ml) were added to mediate a plasma clot. One milliliter of streptokinase (0.1 mg/ml) was used as a plasminogen activator to initiate clot lysis. During the course of lysis, 100 microliters of soluble material were transferred to microtiter wells and the absorbance at 540 nm was determined as a measure of clot lysis. This assay was used to measure clot lysis in 18 human plasma samples. The colorimetric method (X) developed in this report correlated well with that determined using a conventional 125I-fibrinogen method (Y): Y = 0.83X + 7.98 (r = 0.91)