Plasma Lipid Transport Research Articles

The genetic background modifies the effects of the obesity mutation, 'fatty', on apolipoprotein gene regulation in rat liver.

Obesity is associated with disorders of plasma lipid transport in many, but not in all obese subjects. The effects of obesity on the regulation of genes involved in plasma lipid transport may depend on specific mutations causing or contributing to obesity and/or on interactions of a specific obesity mutation with the genetic background. The 'fatty' (Glu269Pro) leptin receptor mutation causes severe obesity associated with hypertriglyceridaemia and altered hepatic apolipoprotein gene regulation in Zucker fatty rats. To determine whether the effects of the obesity mutation 'fatty' on apolipoprotein gene regulation in rat liver depend on the genetic background. We studied hepatic apolipoprotein (apo) A-IV, A-I, and C-III gene expression in obese rats carrying the 'fatty' mutation on the background of the Zucker or Wistar strain. Basal apoA-IV gene expression was increased in fatty rats of both strains, whereas apoA-I and apoC-III gene expression differed between Wistar and Zucker fatty rats: apoA-I gene transcription was reduced to half and apoC-III mRNA was increased two-fold in Wistar fatty, but not in Zucker fatty rats vs lean controls. A fish oil diet suppressed apoA-IV, but not apoA-I gene transcription in Wistar fatty rats, whereas in Zucker fatty rats apoA-IV transcription was unaffected, but apoA-I transcription was suppressed. Interactions of the 'fatty' leptin receptor mutation with the genetic background significantly affect the basal and diet-induced regulation of the apoA-IV, C-III and A-I genes in rat liver. The genetic background may therefore be a major determinant of the consequences of a specific obesity mutation for plasma lipid transport.

Expression and Clinical Significance of Apolipoprotein D in Epithelial Ovarian Carcinomas

Objective. Apolipoprotein D is a protein component of the human plasma lipid transport system but is also associated with a more favorable prognosis in women with breast cancer. This retrospective study was undertaken to examine the tumoral expression of apolipoprotein D in epithelial ovarian cancer and to analyze the possible correlation with tumor and patient characteristics as well as androgen receptors and their prognostic significance.Methods. Immunohistochemical evaluation was used to examine apolipoprotein D expression in paraffin blocks from 68 epithelial ovarian carcinomas.Results. A total of 18 (26.4%) tumors stained positively. No significant correlation was found between apolipoprotein D expression and patient or tumor characteristics and androgen receptor status. However, apolipoprotein D expression was significantly associated with prognosis in patients with residual tumor greater than 1 cm. Thus, patients with apolipoprotein-D-negative tumors had a poorer overall survival than those with apolipoprotein-D-positive tumors (P = 0.039). In addition, multivariate analysis demonstrated that apolipoprotein D expression was an independent prognostic factor with initial tumor size in this group of patients (P = 0.005).Conclusions. Our results led us to consider the existence of apolipoprotein D expression by a significative percentage of ovarian carcinomas, and this protein expression might be of clinical usefulness for identificating lesions with different evolution.

Cytokine and cytokine receptor polymorphisms in infectious disease

Host immune responses strongly control the outcome of infectious disease, and the resistance to bacterial infections in humans is in part genetically determined. Responses to Gram-negative bacterial endotoxin are affected by the concentration of plasma lipoproteins and lipid transport proteins that are genetically controlled. Alterations of plasma lipid profiles by genetic manipulation in mice indeed strongly modify resistance to bacterial infections. Recently, the TLR-4 receptor has been identified as the endotoxin receptor, and TLR-4 mutations are the cause of endotoxin resistance in mice. It is probable that such defects also exist in humans, although they must be rare. The capacity of monocytes to produce TNFalpha varies more than tenfold, and several polymorphisms within the TNFalpha gene have been associated with increased TNFalpha production and increased mortality of sepsis. However, these associations most likely result from linkage disequilibrium with other immune response genes on chromosome 6. Polymorphisms within the IL-1beta and the IL-1RA genes (located on chromosome 2) are associated with altered protein production rates, and certain haplotypes have been linked to inflammatory disease (no studies in bacterial infectious disease have been published). Mutations in the receptors for IL-12 and IFNgamma, both critical for clearance of intracellular infectious pathogens, occur in consanguineous populations and are associated with severe recurrent infections with Salmonella species and mycobacteria. In conclusion, no human cytokine deficiency syndromes are known, and it remains uncertain whether genetically determined differences in the production rate of pro-inflammatory cytokines alter the outcome of sepsis. In contrast, mutations in the IL-12 and IFNgamma receptor (and possibly the endotoxin receptor) genes are associated with recurrent bacterial infections, whereas TNFR1 mutations cause fever of unknown origin.

Apolipoprotein A-I localization and dipalmitoylphosphatidylcholine dynamics in reconstituted high density lipoproteins

The structure and molecular dynamics of recombinant high density lipoproteins (rHDL) were studied by non-radiative energy transfer (NRET), fluorescence anisotropy and intensity measurements. The rHDL particles contained human plasma apolipoprotein (apo) A-I and dipalmitoylphosphatidylcholine (DPPC). Fluorescent cis- and trans-parinaric acids were used both as probes of molecular motion in the particle lipid phase and as acceptors in the Forster’s energy transfer from apo A-I tryptophan residues to determine particle dimensions, apolipoprotein localization and lipid dynamics. The probes are sensitive to thermal wobbling (macromobility) and conformational deformations (micromobility) of phospholipid acyl chains. The experimental data fitted to various models of the particle structure are compatible with the following: (a) at T< T t the particles appeared as lens-like discs with a radius of the lipid phase of 5 nm and a mean thickness of 4 nm, the value being more by 20% in the particle centre, the α-helices of about 1 nm thickness were located around the edge of the lipid core. Compared to liposomes, both macro- and micromobility of DPPC molecules in rHDL were more rapid due to a significant disorder of the boundary lipid molecules close to the apo A-I molecule. This disorder led to the increase of the specific surface area per one lipid molecule, S o. The lipid phase can be divided into three regions: (i) zone I of the most tightly packed lipid (0–1.7 nm from the disc axis) with a S o value small as 0.5 nm 2; (ii) intermediate zone II (from 1.7 to 4.0 nm); and (iii) boundary lipid zone III (4–5 nm) of significantly disordered lipid with a S o value large as 0.65 nm 2. (b) at T>T t the S o heterogeneity disappeared, the radius of the lipid phase did not increase significantly, not exceeding 5.2–5.4 nm, but protein-induced immobilization of lipid molecules which affected about half or more of the total lipid, became remarkable. The overall effect was the supression of the transition amplitude in rHDL compared to liposomes. The structural inhomogeneity might underlie the function of the native plasma HDL as the key component of the transport and metabolism of plasma lipids.

Expression and clinical significance of apolipoprotein D in male breast cancer and gynaecomastia.

Apolipoprotein D (Apo D) is a protein component of the human plasma lipid transport system which is present in benign and malignant human breast tissues. This study analysed the expression of Apo D in men with gynaecomastia or breast cancer, and evaluated its use as a prognostic marker in breast cancer. Immunohistochemical expression of Apo D was examined in specimens from 15 men with gynaecomastia, two with in situ breast carcinoma and 68 with invasive male breast cancer. Median follow-up in patients with breast cancer was 44 months. All gynaecomastia specimens, both in situ carcinomas and 57 invasive carcinomas (84 per cent) stained positively for Apo D. Apo D values were significantly correlated with axillary node involvement and histological grade of the tumours. In men with breast cancer univariate analysis showed a statistical association between node status and Apo D content with relapse-free survival (P < 0.001) and overall survival (P < 0.05). Cox multivariate analysis showed that Apo D was a significant indicator of relapse-free survival (P = 0. 0089), but node status was the strongest factor able to predict both relapse-free (P = 0.0336) and overall (P = 0.0346) survival. Apo D was expressed in gynaecomastia and a high percentage of male breast carcinomas. There was a positive association of Apo D content in male breast tumours with favourable outcome. Apo D expression was a significant independent indicator of relapse-free survival in male breast cancer.

Dietary iron overload and induced lipid peroxidation are associated with impaired plasma lipid transport and hepatic sterol metabolism in rats.

Although hemochromatosis is characterized by dramatic morphological and functional alterations in the liver, little is known about the effects of an excess of iron on lipid metabolism. Therefore, we determined the effect of chronic iron overload on plasma lipid profile and lipoprotein composition, as well as on hepatic cholesterol metabolism and biliary sterol output. Rats administered a diet enriched with 3% iron carbonyl for 12 weeks displayed a 30-fold increase in iron (P <.0001) and a 5-fold rise in malondialdehyde (P <.001) in the liver. When compared with pair-fed controls, iron-overload rats showed a significant increase in triglycerides (P <.005), free cholesterol (P <.006), cholesteryl ester (P <.007), and high-density lipoprotein (HDL)-cholesterol (P <. 003). Triglyceride and cholesteryl ester enrichment, protein depletion, size increase, and apolipoprotein composition alterations characterized the very low density lipoprotein (VLDL) and HDL particles of iron-overload rats. Assessment of the activity of intracellular key enzymes for cholesterol homeostasis in these rats disclosed a reduction in 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (66%, P <.005) and cholesterol 7alpha-hydroxylase (58%, P <.0004) with an increment of acyl-CoA: cholesterol acyltransferase (62%, P <.002). The lack of optimal enzymatic activity may be a result of marked membrane lipid peroxidation that brings about fluidity drop (P <.029) in microsomes of iron-overload rats (5.00 +/- 0.013) versus controls (8.20 +/- 0. 03), reflected by polarization. A decline of the pool size of cholesterol and bile acids was noted in iron-overload rats during a 6-hour bile drainage. Our results show that experimental iron overload causes marked perturbations in plasma lipid transport and hepatobiliary sterol metabolism. Given the positive correlation of malondialdehyde with most of the altered parameters, iron-catalyzed lipid peroxidation may well be one of the involved mechanisms.

Protection against atherogenesis in mice mediated by human apolipoprotein A-IV.

Apolipoproteins are protein constituents of plasma lipid transport particles. Human apolipoprotein A-IV (apoA-IV) was expressed in the liver of C57BL/6 mice and mice deficient in apoE, both of which are prone to atherosclerosis, to investigate whether apoA-IV protects against this disease. In transgenic C57BL/6 mice on an atherogenic diet, the serum concentration of high density lipoprotein (HDL) cholesterol increased by 35 percent, whereas the concentration of endogenous apoA-I decreased by 29 percent, relative to those in transgenic mice on a normal diet. Expression of human apoA-IV in apoE-deficient mice on a normal diet resulted in an even more severe atherogenic lipoprotein profile, without affecting the concentration of HDL cholesterol, than that in nontransgenic apoE-deficient mice. However, transgenic mice of both backgrounds showed a substantial reduction in the size of atherosclerotic lesions. Thus, apoA-IV appears to protect against atherosclerosis by a mechanism that does not involve an increase in HDL cholesterol concentration.

Secretion and processing of apolipoprotein A-I in the avian sciatic nerve during development.

Apolipoprotein A-I (apo A-I), a major apolipoprotein synthesized by liver and intestine to facilitate transport of plasma lipids as lipoproteins, has been detected also in the avian sciatic nerve. The mRNA and protein levels of apo A-I have been shown to increase during the period of rapid myelination (LeBlanc et al.: J Cell Biol 109:1245-1256, 1989). In order to assess the synthesis of apo A-I protein and the processing of apo A-I isoforms during development, endoneurial slices of avian sciatic nerves from chicks during active myelination at 15 and 17 days embryonic and 1 day posthatch age were incubated with [35]S-methionine. The incubations were fractionated into secreted and intracellular fractions, and incorporation of the label was assessed for apo A-I protein. The pattern of labeling of Po protein, as a marker of myelination, was also determined in the intracellular and compact myelin fractions. Methionine incorporation into Po protein was highest in the intracellular compartment at the 15-day embryonic stage and decreased thereafter, with a corresponding increase in the myelin fraction. During these developmental periods, the levels of nascent apo A-I increased in both the secreted and intracellular fractions. The synthesis of apo A-I specifically increases in the secreted fraction compared with total protein synthesis. The processing of the pro-apo A-I is also developmentally regulated. In the intracellular compartment, there are approximately equal proportions of the acidic and basic isoforms. However, with increasing age, a higher proportion of the apo A-I is secreted as acidic isoforms. It is concluded that the secretion and processing of apo A-I is developmentally regulated in the chick sciatic nerve, in parallel with the process of active myelination.

1] Apolipoproteins: Pathophysiology and clinical implications

The term apolipoprotein was initially coined by John Oncley to refer to the protein constituents of plasma lipoproteins. Plasma apolipoproteins are of interest principally because of their central role in plasma lipid transport and, hence their role in atherogenesis. Over the past several decades, a great deal has been learned about apolipoprotein structure and function. The regulation of apolipoprotein gene expression and the molecular mechanisms of apolipoprotein function are beginning to be revealed. Given the rate of progress in this research, therapeutic applications that target the expression or metabolism of specific apolipoproteins can be expected in the near future. Central to the functions of all apolipoproteins (apo) is the ability to bind phospholipids that reside in specialized regions termed “amphiphathic helices.” The characteristic feature of the amphipathic helix is the spatial arrangement of hydrophobic and hydrophilic amino acids. The hydrophobic face of the helix is intercalated between the fatty acyl chains of phospholipids, whereas the hydrophilic face is located close to the polar head groups of phospholipids. Such an orientation allows the interaction of protein domains with lipoprotein-modifying enzymes and cellular receptors that control the catabolism of lipoproteins (Lp) and their removal from the circulation. The apoB gene differs from other apolipoprotein genes in genomic organization, in that it contains 29 exons, 2 of which are extremely long. In addition to the amphipathic helix motifs, located mainly in the carboxyterminal half, apoB contains unique proline-rich sequences predicted to form amphipathic β sheets. The evolutionary relationship between apoB and the other apolipoproteins is, therefore, not clear. ApoD is not a member of the apolipoprotein gene family, as it differs from all other apolipoproteins in genomic organization, primary structure, and tissue sites of expression. Similar arguments can be used to qualify apo[a] and minor apolipoproteins, such as aport and apoJ, as atypical apolipoproteins.

The role of stable isotopes in the investigation of plasma lipoprotein metabolism

The last 5 years have seen promising beginnings of the application of stable-isotope-based methods to the study of lipoprotein metabolism. Many aspects of plasma lipid transport make it an attractive system for investigating by this means. While early efforts borrowed from standard techniques of generating and interpreting kinetic parameters (FSRs), the shortcomings of these procedures as applied to lipoproteins are now appreciated. Lipoprotein heterogeneity requires that multicompartmental analysis and relatively long-term studies be employed if the information obtained is going to be a useful adjunct to that produced by radio-iodinated lipoprotein tracers. Both approaches-stable-isotope-labeled endogenous tracer and ex vivo radio-iodinated lipoprotein experiments--must be considered complementary. The first provides direct information on lipoprotein synthesis pathways while the latter is superior at following interconversions and catabolic events. Excellent agreement has been demonstrated where the methods have been used simultaneously to estimate the same kinetic parameter. Many of the questions that have arisen from decades of radioactive tracer studies relate to the nature and rate of lipoprotein synthesis, e.g. what kinds of particles are produced by the liver when different diets are taken, and how are the synthetic pathways altered in dyslipidaemic states? Endogenous tracers can address these issues, and methods are presently available which provide the means for measuring the production of all apolipoproteins and for estimating cholesterol and fatty acid biosynthesis. When techniques are developed to examine triglyceride, cholesteryl ester and phospholipid production then a much clearer picture of how lipoproteins are assembled in vivo will emerge. This kind of information will be essential to an understanding of regulation of plasma lipid transport and the subtle changes that occur in those at risk for coronary heart disease.

Plasma lipid transport in the hedgehog: partial characterization of structure and function of apolipoprotein A-I.

Apart from exhibiting the presence of lipoprotein [a] in its plasma, another interest of the European hedgehog in lipoprotein research lies in the quantitative prominence of a complex spectrum of high density lipoproteins (HDL) and very high density lipoproteins (VHDL) as cholesterol transporters in plasma (Laplaud, P. M. et al. 1989. Biochim. Biophys. Acta. 1005: 143-156). We, therefore, initiated studies in the field of reverse cholesterol transport in the hedgehog. As a first step, we characterized apolipoprotein A-I (apoA-I), the main protein component of hedgehog HDL and VHDL. Proteolytic cleavage of apoA-I (M(r) approx. 27 kDa) using two different enzymes resulted in two sets of peptides that were subsequently purified by high performance liquid chromatography, and that allowed us determination of the complete protein sequence. Hedgehog apoA-I thus consists of 241 amino acid residues and exhibits an overall 58% homology to its human counterpart, i.e., the lowest value observed to date among mammalian species. However, it retained the general organization common to all known apoA-Is, i.e., a series of amphipathic helical segments punctuated by proline residues. Circular dichroism experiments indicated a helical content of approx. 45%, increasing to approx. 58% in the presence of lecithin unilamellar liposomes. Apart from other differences, amino acid composition analysis shows that hedgehog apoA-I contains four isoleucine residues, while this amino acid is totally absent from the corresponding protein in higher mammals. Polyclonal antibodies raised against hedgehog apoA-I failed to detect any cross-reactivity between the animal and human proteins, although comparative prediction of the respective antigenic structures using the Hopp-Woods algorithm indicated that several potentially antigenic sites may occur in similar regions of the protein. Finally, hedgehog apoA-I was shown to be able to activate lecithin:cholesterol acyl transferase, although it was 4 to 5 times less efficient in this respect than the human protein.

Cholesteryl ester transfer protein: its role in plasma lipid transport.

1. The cholesteryl ester transfer protein (CETP) is a hydrophobic glycoprotein which acts in plasma to redistribute cholesteryl esters and triglyceride between plasma lipoproteins. 2. CETP also plays an important role in determining the composition and particle size distribution of high density lipoproteins (HDL). 3. Activity of CETP may be regulated in four ways: By factors which influence the concentration of CETP in plasma; by the activity of CETP inhibitor proteins; by variations in the concentrations and compositions of donor and acceptor lipoproteins and by factors which influence the interaction of CETP with plasma lipoproteins. 4. The mechanism of action of CETP is uncertain. Two models have been proposed: (i) a shuttle model in which CETP physically transports lipids between lipoprotein particles and (ii) a ternary complex model in which CETP forms a bridge between two lipoprotein particles, enabling them to exchange lipids. 5. Evidence is accumulating that CETP may be a pro-atherogenic factor.

Associations of allelic differences at the A-I/C-III/A-IV gene cluster with carotid artery intima-media thickness and plasma lipid transport in hypercholesterolemic-hypertriglyceridemic humans.

Individuals with elevated levels of plasma cholesterol and triglyceride may be at higher risk for coronary artery disease than those with isolated elevations of either cholesterol or triglyceride. Sequence variation in the A-I/C-III/A-IV gene cluster has been implicated in the etiology of some disorders associated with premature atherosclerosis and/or hypertriglyceridemias with or without elevations of cholesterol. This led to the hypothesis that allelic variation at this gene locus alters plasma lipid transport and affects susceptibility for atherosclerosis. The study population, from the Atherosclerosis Risk in Communities (ARIC) Study, consisted of 50 normolipidemic individuals, 48 subjects with elevated plasma cholesterol, 47 subjects with elevated plasma triglyceride, and 123 subjects with both elevated plasma cholesterol and triglyceride who were used to evaluate associations between an Xmn I polymorphic site 2.5 kilobase pairs (kbp) upstream of the structural gene for apolipoprotein (apo) A-I, intimal-medial thickening of the extracranial carotid arteries, and several plasma lipid factors. The relative allele frequencies of the 8.3-kbp allele and the 6.6-kbp allele were .86 and .14, respectively, in the entire study population and did not differ among the lipid phenotypes. In the group with elevated plasma cholesterol and triglyceride, subjects possessing the 6.6-kbp allele exhibited a greater carotid artery intimal-medial thickness (P = .034) and higher plasma levels of apoA-I, high-density lipoprotein (HDL) cholesterol, and HDL3 cholesterol (P < .02) than subjects homozygous for the 8.3-kbp allele. In contrast, subjects with the 6.6-kbp allele displayed lower mean ratios of apolipoproteins C-II to C-III, C-II to A-IV and E to A-IV in plasma (P < .05) and a lower mean ratio of apolipoprotein C-II to C-III in the triglyceride-rich lipoproteins (P = .026). Sequence variation in or near the genes encoding apolipoproteins A-I, C-III, and A-IV may therefore identify a group of hypercholesterolemic-hypertriglyceridemic persons who are at higher risk for atherosclerosis than others with the same lipoprotein phenotype.

Kinetics of lipids, apolipoproteins, and cholesteryl ester transfer protein in plasma after a bicycle marathon

The short-term effects of prolonged intense exercise on plasma lipid transport parameters including cholesterol, triglycerides (TGs), low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and its subfractions HDL 2 cholesterol and HDL 3 cholesterol, on apolipoproteins (apos) A-I, A-II, and B, and on mass and activity of cholesteryl ester transfer protein (CETP) were studied in eight male endurance-trained athletes over the first week after a bicycle marathon. CETP mass concentration in plasma was quantified by a newly developed immunoradiometric assay (IRMA). Plasma concentrations of cholesterol, TGs, LDL cholesterol, apo B, CETP, and cholesteryl ester transfer activity (CETA) were significantly reduced in the recovery period compared with pre-exercise values (cholesterol by 20%, P < .05; TGs by 63%, P < .05; LDL cholesterol by 32%, P < .05; apo B by 18%, P < .05; CETP mass by 29%, P < .05; and CETA by 14%, P < .05). HDL cholesterol and HDL 2 cholesterol, in contrast, were significantly increased in the post-exercise period (HDL cholesterol by 12%, P < .05, and HDL 2 cholesterol by 96%, P < .05), whereas HDL 3 cholesterol showed a tendency to decrease in the late recovery period (by 8%, NS). Although changes in cholesterol, triglycerides, HDL cholesterol, LDL cholesterol, apo B, and CETP mass and activity were already evident in the early recovery period, HDL 2 cholesterol showed a delayed response, reaching its maximum 72 hours after initiation of exercise. In addition, significant increases in plasma levels of apo A-I and A-II were found 8 days after the marathon (by 5%, P < .05, and by 12%, P < .05, respectively). Our data suggest that even in highly trained athletes, universally favorable lipoprotein changes of unexpected quantity result from a single episode of heavy exertion. The sustained and pronounced increase of HDL 2 cholesterol may be explained at least in part by the decrease in CETP after short-term exercise.

Plasma lipid transport in the horse ( Equus caballus)

1. 1. Equine plasma contains lipoproteins corresponding to very low density (VLDL), low density (LDL) and high density lipoproteins (HDL). 2. 2. HDL accounts for approximately 60% of plasma lipoprotein mass and consists of a single population of particles. 3. 3. LDL is heterogeneous comprising three discrete subfractions. 4. 4. Two proteins are found in the region of apolipoprotein (apo) B-100 in VLDL and LDL and a third similar to apo B-48 is in VLDL. 5. 5. Lecithin:cholesterol acyl transferase is active in plasma and hepatic lipase and lipoprotein lipase are evident in post-heparin plasma. 6. 6. There is no significant cholesteryl ester transfer protein activity.

Lipoprotein Metabolism

The pathways of cellular synthesis, assembly, and secretion of lipoproteins followed by their subsequent intravascular metabolism and cellular uptake provide an efficient system for the transport of exogenous and endogenous lipids. The present report reviews the pathways of normal lipoprotein metabolism and the roles played by specific apoproteins, transfer proteins, enzymes, and cellular receptors in facilitating the transport of lipids in plasma, as well as in the regulation of cellular cholesterol homeostasis.

Transcription efficiency of human apolipoprotein A-I promoter varies with naturally occurring A to G transition

In human, the gene coding for apolipoprotein A-I (apo A-I), a protein of the plasma lipid transport system, is expressed only in the liver and the intestine. A naturally occurring A to G substitution in the promoter at position −78 was shown to be associated with high density lipoproteins (HDL) in females. We have studied the effect of this mutation on promoter activity using various lengths of promoter sequences and the CAT reporter gene system. Transient expression studies after introduction of these constructs into Hep 3B cells revealed that in the region spanning −330 to +1 of the promoter an A to G substitution increases the activity approximately twofold. On the other hand, when further upstream region (−1469 to +397) is also included, the promoter activity seems comparable in both alleles. Our results show how minimal sequence variations can affect the in vitro analysis of promoter activity.

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.

Obesity and lipid metabolism

This paper reviews the recently published literature on obesity, body fat distribution and lipid metabolism. Studies have emphasized the importance of visceral fat as a correlate of alterations in plasma lipid transport observed in abdominal obesity. Some of the mechanisms in the relation of fat dis

Role of Apolipoprotein Levels in Clinical Practice

Enormous advances have been made in our understanding of the role of apolipoproteins in the transport of plasma lipids. The apolipoproteins are essential for the packaging of lipoproteins in the liver and intestine, for the interconversion of lipoproteins in the circulation, and for their uptake by tissues. In a sense, the apolipoproteins are the "business end" of the lipoproteins. Since the serum lipoproteins undoubtedly play an important role in atherogenesis, it is reasonable to ask whether serum levels of apolipoproteins can be used as predictors of coronary heart disease (CHD) in the same way that serum cholesterol levels are used for this purpose. The two apolipoproteins that have received the most attention are apolipoprotein B and apolipoprotein A-I. Consideration might be given to the potential value of measurement of each.