The primary structure of apolipoprotein A-I from rabbit high-density lipoprotein

Generating a detailed protein profile of Fasciola hepatica during the chronic stage of infection in cattle

Fasciola hepatica is a trematode helminth causing a damaging disease, fasciolosis, in ruminants and humans. Comprehensive proteomic studies broaden our knowledge of the parasite's protein profile, and provide new insights into the development of more effective strategies to deal with fasciolosis. The objective of this study was to generate a comprehensive profile of F. hepatica proteins expressed during the chronic stage of infection in cattle by building on previous efforts in this area. The approach included an improved sample preparation procedure for surface and internal layers of the parasite, the application of nano-UPLC-ESI-qTOF-MS (nano-ultra-performance LC and ESI quadrupole TOF MS) integrated with different acquisition methods and in silico database search against various protein databases and a transcript database including a new assembly of publically available EST. Of a total of 776 identified proteins, 206 and 332 were specific to the surface and internal layers of the parasite, respectively. Furthermore, 238 proteins were common to both layers, with comparative differences of 172 proteins detected. Specific proteins not previously identified in F. hepatica, but shown to be immunomodulatory or potential drug targets for other parasites, are discussed.

Conformation and stability properties of B17: II. Analytical investigations using differential scanning calorimetry

Thermal and stability properties of B17, the 17% N-terminal domain of apo B, were carried out using differential scanning calorimetry spectroscopy, where the thermal characteristics of the polypeptide were studied and analyzed. The heat capacity data of B17 showed that the protein undergoes two transitions between 50 and 90 °C, with T m's at 65.9 and 74.8 °C. While the first transition showed immediate reversibility, the second one-with the higher T m-necessitated a longer cooling (several days) period for its reversibility to be observed and both transitions could be seen in the heat capacity profile of B17. Moreover, the van't Hoff enthalpies determined via calorimetric measurements agreed with the values calculated from the CD analysis reported previously.

Conformation and stability properties of B17: I. Analytical investigations using circular dichroism

Structural characterization of B17, the 17% N-terminal domain of apo B, was carried out using circular dichroic (CD) spectroscopy, where secondary and tertiary structures were studied as a function of temperature and pH. Mild acidic conditions that correlate with histidine protonation invoked a change in the α-helix and random coil contents of the protein, with no apparent change in the β-sheet structural content. Specific changes in the structure of the protein that occur in response to temperature were also investigated to understand the stability and conformational changes of B17. Far- and near-UV CDs were used to probe the thermal changes in the protein. The protonation of some histidine residues was attributed to underlie the increase in the helical content of the protein.

Plasma kinetics of cholesteryl ester transfer protein in the rabbit. Effects of dietary cholesterol

The plasma kinetics of recombinant human cholesteryl ester transfer protein (rCETP) were studied in six rabbits before and after cholesterol feeding (0.5% wt/wt). The rCETP, labeled with the use of the Bolton Hunter reagent, was shown to retain neutral lipid transfer activity. After intravenous infusion, labeled rCETP associated with rabbit lipoproteins to an extent similar to endogenous rabbit CETP (62% to 64% HDL associated). The plasma kinetics of CETP, modeled with the use of SAAM-II, conformed to a two-pool model, likely representing free and loosely HDL-associated CETP (fast pool) and a tightly apo (apolipoprotein) AI-associated (slow pool) CETP. The plasma residency time (chow diet) of the fast pool averaged 7.1 hours and of the slow pool, 76.3 hours. The production rate (PR) into and the fractional catabolic rate (FCR) of the fast pool were 20 and 10 times the PR and FCR, respectively, of the slow pool. In response to cholesterol feeding, CETP PR, FCR, and plasma mass increased by 416%, 60%, and 230%, respectively. There was a strong correlation (r = .95, P = .003) between the increase in rabbit plasma CETP and the modeled increase in CETP PR in response to cholesterol feeding, suggesting that labeled human rCETP is a satisfactory tracer for rabbit plasma CETP. CETP is catabolized by distinct pools, likely corresponding to an apo AI-associated (slow) pool and a free and/or loosely HDL-associated (fast) pool. Factors that alter the affinity of CETP for HDL would be predicted to result in altered CETP catabolism. The effect of dietary cholesterol on plasma CETP mass can be explained largely by the effects on CETP synthesis, consistent with the observed effects of cholesterol on tissue mRNA levels.

Intravenous injection of rabbit apolipoprotein A-I inhibits the progression of atherosclerosis in cholesterol-fed rabbits

The effects of intravenous injection of purified rabbit apoA-I on the progression of aortic atherosclerosis in cholesterol-fed rabbits were examined. In experiment 1, 28 rabbits were equally divided into groups A and B and fed a 0.5% cholesterol diet for 90 days. For the last 30 days, group B received 40 mg apoA-I every week. The fatty streak lesions in group B (23.9 +/- 15.6%) were significantly suppressed compared with those in group A (46.0 +/- 24.9%) (P < .05). In experiment 2, 33 rabbits were divided into four groups (8 or 9 rabbits per group) and fed a 0.5% cholesterol diet. Group A was killed on day 105, while groups B, C, and D were maintained for an additional 60 days on a normal diet, during which time groups C and D received 1 mg apoA-I every other day or 40 mg apoA-I every week, respectively. The lesions in group C (70.2 +/- 15.4%) and group D (65.7 +/- 20.0%) were significantly suppressed compared with those in group B (86.2 +/- 13.7%) (P < .05) but were not reduced to the level of group A (50.0 +/- 22.9%). Although apparent regression was not observed under these conditions, the present study provided the first evidence for the antiatherogenic effect of homologous and apoA-I on the progression of atherosclerosis in cholesterol-fed rabbits.

Primary structure of Beijing duck apolipoprotein A-1

The primary structure of Beijing duck apolipoprotein A-1 was determined by sequencing peptide fragments derived from tryptic and endoproteinase Asp-N digestion of the protein, and alignment with homologous chicken apo A-1. All of the peptide fragments were isolated by high-pressure liquid chromatography (HPLC) with a Vydac C18 column using a trifluoroacetic acid (TFA) buffer system. The N-terminus of the protein was determined to be aspartic acid by directly sequencing 52 residues of the intact protein. The C-terminus was alanine. The protein contains 240 amino acid residues. By analysis of the whole protein and its tryptic peptides, a six amino acid (Arg-Tyr-Phe-Trp-Gln-His) prosegment was determined. No cross-reactivity between duck and human apo A-1 with a goat antiserum against human apo A-1 was found. Sequence analysis of apo A-1 of other species indicates that amino acid substitutions in rat are more extensive than in other mammals. Isoleucine residues in apo A-1 are inversely correlated to the homology of human to other species, except dog.

Plasma lipid transport in the preruminant calf, Bos spp: primary structure of bovine apolipoprotein A-I

The preruminant calf (Bos spp.) is a model of considerable interest with regard to hepatic and intestinal lipoprotein metabolism (Bauchart et al., J. Lipid Res. (1989) 30, 1499-1514 and Laplaud et al., J. Lipid Res. (1990) 31, 1781-1792). As a preliminary step towards future experiments dealing with HDL metabolism in the calf, we have purified apoA-I from this animal and determined its complete amino acid sequence. Thus, approx. 10% of calf apoA-I was shown to contain a propeptide, with the sequence Arg-His-Phe-Trp-Gln-Gln. Enzymatic cleavage of apoA-I resulted in 10 proteolytic peptides. The complete apoA-I sequence was obtained after alignment of peptides on the basis of their homologies with those from rabbit apoA-I. Thus calf apoA-I consists of 241 amino acid residues, and exhibits high sequence homology with all mammalian apoA-I's studied to date. The bovine protein contained 10 hydrophobic amphipathic helical regions, occurring between residues 43-64, 65-86, 87-97, 98-119, 120-141, 142-163, 164-184, 185-206, 207-217 and 218-241. A computer-constructed phylogenetic tree showed that bovine apoA-I was more closely related to its dog counterpart, including the presence of a single methionine, than to the corresponding macaque and human proteins. Comparative predictions of the respective antigenic structures of human and bovine apoA-I's using the Hopp-Woods algorithm indicated similar positions for all 13 detectable antigenic sites, among which 7 were of identical, or closely related, amino acid composition. This finding was confirmed by demonstration of partial immunological identity between the two proteins upon immunodiffusion analysis, a result obtained using a monospecific rabbit antiserum against bovine apoA-I. Finally, comparison of sequence homology between bovine apoA-I and the lecithin:cholesterol acyl transferase (LCAT) activating region of human apoC-I suggests that several LCAT activating domains may be present in calf apoA-I.

Characterization and amino-terminal sequence of apolipoprotein AI from plasma high density lipoproteins in the preruminant calf, Bos spp

The major apolipoprotein of calf plasma high-density lipoproteins, apo-AI, has been isolated and characterized. Apolipoprotein AI (apo-AI) was separated from the protein moiety of high-density lipoproteins (d 1.090-1.180 g/ml) by preparative electrophoresis in SDS-polyacrylamide gels followed by electrophoretic elution. Purified calf apo-AI had an Mr of approx. 27,000-28,000 in SDS-polyacrylamide gels, resembling human apo-AI. The amino acid composition of calf apo-AI displayed an overall similarity to that of its human and other mammalian counterparts (baboon, dog, badger, rabbit, rat and mouse), but differed in having higher proportions of glutamic acid, alanine and isoleucine. Amino-terminal amino acid sequence analysis up to the 47th residue showed close homology between calf apo-AI and those of the mammals with which it was compared. However, residues 2, 7, 20 and 22 in calf AI (i.e. aspartic acid, serine, glutamic acid and isoleucine, respectively) were substituted by glutamic acid, proline or glutamine, aspartic acid, and valine or leucine respectively, in the other mammals.

Determination of cysteine on low-density lipoproteins using the fluorescent probe, 5-iodoacetamidofluoresceine

Using the fluorescent sulfhydryl probe, 5-iodoacetamidofluoresceine, to label the free sulfhydryl of low-density lipoprotein, the positions of two cysteine residues in apolipoprotein B were located. The tryptic peptides containing the fluorescent probe were isolated by high-performance liquid chromatography systems and sequenced by automatic techniques. The free cysteine residues of apolipoprotein B-100 on low-density lipoprotein are located at positions 3734 and 4190, either or both of which can potentially form a disulfide linkage with apolipoprotein(a) in lipoprotein(a).

Use of synthetic peptide analogues to localize lecithin:cholesterol acyltransferase activating domain in apolipoprotein A-I

The major protein of high density lipoprotein (HDL), apolipoprotein (apo) A-I, is the major activator of the plasma enzyme lecithin:cholesterol acyltransferase (LCAT). A consensus amino acid sequence has been defined for the eight, 22-residue long, tandem amphipathic helical repeats located in the carboxy-terminal region of apo A-I. A series of 22 and 44mer synthetic peptide analogues of the consensus domain, differing only in their 13th amino acid residue, were prepared and tested for LCAT activation. One of the peptides was found to equal apo A-I in LCAT activation. This is the first time a peptide activator for LCAT that rivals the activity of apo A-I in the vesicular and discoidal egg phosphatidylcholine assay systems has been synthesized. Based on these results, we propose that the major LCAT-activating domain of apo A-I resides in the 22mer tandem repeats, each containing Glu at the 13th residue and located between residues 66 and 121 in the native apolipoprotein.

Apolipoprotein A-I variants. Naturally occurring substitutions of proline residues affect plasma concentration of apolipoprotein A-I

Six unrelated families with genetically determined structural variants of apo A-I were found in the course of an electrophoretic screening program for apo A-I variants in dried blood samples of newborns. The following structural variations were identified by the combined use of HPLC, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and automated gas phase sequencing: Pro3----Arg (1x), Pro4----Arg (1x), and Pro165----Arg (4x). All variant carriers were heterozygous for their mutant of apo A-I. Subjects heterozygous for apo A-I(Pro165----Arg) (n = 12) were found to exhibit lower mean values for apo A-I (109 +/- 16 mg/dl) and HDL cholesterol (37 +/- 9 mg/dl) than unaffected family members (n = 9): 176 +/- 41 and 64 +/- 18 mg/dl, respectively (P less than 0.001). In 9 of 12 apo A-I(Pro165----Arg) variant carriers the concentrations of apo A-I were below the fifth percentile of sex-matched controls. By two-dimensional immunoelectrophoresis as well as by densitometry the relative concentration of the variant apo A-I in heterozygous carriers of apo A-I(Pro165----Arg) was determined to account for only 30% of the total plasma apo A-I mass instead of the expected 50%. Thus, the observed apo A-I deficiency may be largely a consequence of the decreased concentration of the variant apo A-I. In the case of the apo A-I(Pro3----Arg) mutant, densitometry of HDL apolipoproteins demonstrated a distinctly increased concentration of the variant proapo A-I relative to normal proapo A-I. This phenomenon was not observed in the apo A-I(Pro4----Arg) mutant or in other mutants. This suggests that the interspecies conserved proline residue in position 3 of mature apo A-I is functionally important for the regular enzymatic conversion of proapo A-I to mature apo A-I.

Expression, location and cross-reactivity of two antigenic sites on the amino terminal region of rabbit and human apolipoprotein A-I

Rabbit apolipoprotein A-I (apo A-I) of molecular weight 27,612 contained 241 amino acids. In contrast to its human counterpart which has 3 methionine residues, the rabbit protein possesses only one and therefore produces 2 fragments after CNBr cleavage (CNBr I and II, NH2- and COOH-terminal, respectively). From a series of monoclonal antibodies raised against human apo A-I, 2 (A05 and A16) cross-reacted with rabbit apo A-I. In the present study, we show that A05 recognizes the rabbit CNBr I fragment while the integrity of the intermediate region between the 2 CNBr fragments (including the methionine residue) is required for the expression of the A16 antigenic determinant. Competition experiments were performed between human 125I-labelled high density lipoprotein (HDL) and a variety of preparations of human and rabbit apo A-I (including the purified and delipidated protein, complexes of dimyristoylphosphatidylcholine (DMPC) containing apo A-I, HDL subfractions and whole serum). The A05 antigenic determinant was expressed identically in all these fractions of both species. In contrast the A16 showed poor reactivity with delipidated apo A-I, the apparent affinity constant being about 100 times less than for HDL. These data suggest that phospholipids improve the recognition of apo A-I by the A16 antibody. The similar immunoreactivity of the human and rabbit proteins in the present study is consistent with the view that the NH2-terminal region contains the major portion activating lecithin:cholesterol acyltransferase.

Characterization of two monoclonal antibodies to human apolipoprotein A-I that also bind to rabbit apolipoprotein A-I. Application to ELISA evaluation of rabbit serum apo A-I

Two monoclonal antibodies against human apolipoprotein A-I were characterized to recognize rabbit apo A-I. By immunoblotting we determined that while neither antibody reacted with the other rabbit apolipoproteins they both recognized all rabbit apo A-I isoforms. Cotitration revealed that each antibody bound to different apo A-I epitopes. Then we developed a noncompetitive enzyme-linked immunosorbent assay (ELISA) to measure the concentration of total apolipoprotein A-I in rabbit serum. The ELISA curves of the different lipoprotein class and serum showed the similarity of the expression of the apo A-I epitopes recognized in each of them and in serum. This assay, as opposed to other techniques, offers several advantages such as sensitivity, specificity, and simplicity and avoids the use of radioisotope.

Rabbit apolipoprotein A-I mRNA and gene. Evidence that rabbit apolipoprotein A-I is synthesized in the intestine but not in the liver

In order to study the tissue-specific expression of rabbit apolipoprotein (apo) A-I, a 923-base-pair clone, pRBA-502, complementary to rabbit apo A-I mRNA was identified from a rabbit intestinal cDNA library by hybrid-select translation and immunoprecipitation methods. Northern blot and dot-blot hybridization, utilizing 32P-labeled pRBA-502, revealed that the rabbit apo A-I gene is expressed in the intestine, not in the liver and that rabbit apo A-I mRNA is about 950 nucleotides in length. The entire nucleotide sequence of pRBA-502 has been determined and the complete amino acid sequence of the corresponding apo A-I has been deduced. The mRNA codes for a protein comprising 265 amino acids. Amino acids 1-18 and 19-24 of the primary translation product represent the presegment and prosegment, respectively, of apo A-I. Matured rabbit apo A-I contains 241 amino acids and has a molecular mass of 27612 Da. Using pRBA-502 as a probe, a 15.5-kb genomic fragment, which contains the entire apo A-I gene, was isolated from a rabbit liver genomic library. Sequence analysis of the gene shows that the 200 base pairs of the 5' upstream flanking region of the rabbit and human apo A-I genes showed 78% sequence homology. Like the human apo A-I gene, the rabbit apo A-I gene is interrupted by three intervening sequences. Except for two nucleotides in the fourth exon, the coding sequence of the rabbit liver apo A-I gene is identical to that of pRBA-502. Our data showed that the lack of expression of apo A-I gene in rabbit liver is not due to the alternation of rabbit liver apo A-I gene sequence and suggest that the expression of apo A-I gene in rabbit liver is regulated by a trans-acting regulating element(s).

The complete amino acid sequence of proapolipoprotein A-I of chicken high density lipoproteins

The complete amino acid sequence of proapolipoprotein (proapo) A-I of chicken high density lipoproteins was determined by sequencing overlapping peptides produced by trypsin, S. aureus V8 protease, and cyanogen bromide cleavage. There are 240 amino acid residues in mature chicken apoA-I. By direct sequence analysis of a cyanogen bromide peptide, we also determined the sequence of a 6-amino-acid prosegment which is present at approx. 10% the molar amount of the mature peptide in chicken plasma. Sequence comparison among apoA-I from chicken, human, rabbit, dog and rat, and secondary structure analysis indicate that while the degree of sequence homology is only moderate (less than 50% between chicken and man), there is good conservation of apoA-I secondary structure, especially in the N-terminal two-thirds of the protein in these widely separated species.