Lipoprotein characteristics

Lipoprotein characteristics depends on the varying proportions of protein and lipid present. Lipids are of lower density (<0.9 g/ml) than proteins (>1.28 g/ml), giving a range of lipoprotein densities from 0.9 to 1.21 g/ml. Lipoprotein classification is made according to the respective densities as determined by ultracentrifugation or according to their electrophoretic migration. 

KAPLAN M and AVIRAM M (1999) Oxidized low density lipoprotein atherogenic and proinflammatoiy characteristics during macrophage foam cell formation. An inhibitory role for nutritional antioxidants and serum paraoxonase Clinical Chemistry Laboratory Medicine 37,111-9,1. 

Recently, a gene coding for a novel pectin methylesterase, has been cloned (19). This gene, pemB, codes for a 433 amino add protein induding a N-terminal sequence of 21 amino adds which presents the characteristics of lipoprotein... 

Up to now, the pectinolytic enzymes of E. chrysanthemi that have been detected were extracellular secreted enzymes (PelA, B, C, D, E, L, exo-Peh and PemA), periplasmic (exo-Pel), or cytoplasmic (OGL) proteins (1, 5). In contrast, PemB is an outer membrane pectinolytic enzyme. To our knowledge it is the first pectinase characterised as a membrane protein. We presented several lines of evidence showing that PemB is a lipoprotein (i) Its N-terminal sequence has the characteristics of lipoprotein signal sequences, (ii) PemB is synthesised as a high molecular weight precursor processed into a lower molecular weight mature form, (iii) Palmitate, the most prevalent fatty acid in bacterial lipoproteins (12), is incorporated into PemB. 

Patients with abetalipoproteinaemia, a rare inborn disorder of lipoprotein metabolism, are totally deficient in vitamin E fiom birth and, if untreated, invariably develop a characteristic pigmentary retinopathy similar to that seen in retinitis pigmentosa and peroxisomal disorders. The same retinopathy has been observed in other patients with severe and chronic vitamin E deficiency. A essive vitamin E replacement therapy in all these patients has been shown either to prevent, to halt the progression of, or in some cases, to improve the characteristic visual abnormalities (Muller and Lloyd, 1982). 

It needs to be noted that apart from expression of lipoprotein receptors, RPE itself expresses several apolipoproteins (Bartl et al., 2001 Ishida et al., 2004 Li et al., 2006 Malek et al., 2003 Tserentsoodol et al., 2006a). So far, six apolipoproteins have been identified as being expressed by the RPE, namely, apolipoprotein A-I (ApoA-I), ApoB, ApoC-I, ApoC-II, ApoE, and ApoJ (clusterin) (Bailey et al., 2004 Bartl et al., 2001 Ishida et al., 2004 Li et al., 2006 Malek et al., 2003 Tserentsoodol et al., 2006a). In addition to their functions as lipid transporters and receptor ligands, apo-lipoproteins can act as modulators of several enzymes. The basic characteristics of apo-lipoproteins expressed by the RPE are described below. 

An important characteristic of mammalian 15-LOX is its capacity to oxidize the esters of unsaturated acid in biological membranes and plasma lipoproteins without their hydrolysis to free acids. Jung et al. [19] found that human leukocyte 15-LOX oxidized phosphatidylcholine at carbon-15 of the AA moiety. Soybean and rabbit reticulocyte 15-LOXs were also active while human leukocyte 5-LOX, rat basophilic leukemia cell 5-LOX, and rabbit platelet 12-LOX were inactive. It was suggested that the oxygenation of phospholipid is a unique property of 15-LOX. However, Murray and Brash [20] showed that rabbit reticulocyte... 

Lipidated peptides embodying the characteristic linkage region found in the parent lipoproteins and bearing additional functional groups, which could be traced in biological systems or which allowed for their use in biophysical experiments, were used successfully in model studies. However, such model studies only provide a limited amount of information. In order to approximate the situation in a biological system more precisely, experiments with differently lipidated proteins are required. 

Whether or not they are lipoproteins, both periplasmic proteins and outer membrane proteins translocate across the inner membrane thus there should be some cellular mechanisms that sort them. Unlike inner membrane proteins, outer membrane proteins do not have characteristic hydrophobic transmembrane segments as such, most, if not all, of them are thought to be composed of ft strands. Moreover, it has been suggested that such conformation may be the determinant of the integration into the outer membrane in other words, these proteins may be spontaneously integrated into the outer membrane. If this assumption is correct, the outer membrane proteins must fold at the periplasm. Another possibility is that the outer membrane proteins are integrated at certain sites where the inner and outer membranes are contacted. This issue has not been solved, but a recent experiment supports the periplasmic folding (Eppens et al., 1997). 

It, thus, appears that the capacity to catalyze reactions of transesterification and esterification is a characteristic of various hydrolases (Chapt. 3). Apart from the carboxylesterases discussed here, lipoprotein lipase has the capacity to synthesize fatty acid ethyl esters from ethanol and triglycerides, or even fatty acids [127]. Ethanol, 2-chloroethanol, and other primary alcohols serve to esterify endogenous fatty acids and a number of xenobiotic acids [128-130]. In this context, it is interesting to note that the same human liver carboxylesterase was able to catalyze the hydrolysis of cocaine to benzoylecgonine, the transesterification of cocaine, and the ethyl esterification of fatty acids [131]. 

The increased degradation of fat that occurs in insulin deficiency also has serious effects. Some of the fatty acids that accumulate in large quantities are taken up by the liver and used for lipoprotein synthesis (hyperlipidemia), and the rest are broken down into acetyl CoA. As the tricarboxylic acid cycle is not capable of taking up such large quantities of acetyl CoA, the excess is used to form ketone bodies (acetoacetate and p-hydroxy-butyrate see p. 312). As H"" ions are released in this process, diabetics not receiving adequate treatment can suffer severe metabolic acidosis (diabetic coma). The acetone that is also formed gives these patients breath a characteristic odor. In addition, large amounts of ketone body anions appear in the urine (ketonuria). 

The answer is a. (Hardman, pp 875-898.) In type I hyperlipoproteinemia, drugs that reduce levels of lipoproteins are not useful, but reduction of dietary sources of fat may help. Cholesterol levels are usually normal, but triglycerides are elevated. Maintenance of ideal body weight is recommended in all types of hyperlipidemia. Clofibrate effectively reduces the levels of VLDLs that are characteristic of types 111, IV, and V hyperlipoproteinemia administration of cholestyramine resin and lovastatin in conjunction with a low-cholesterol diet is regarded as effective therapy for type 11a, or primary, hyperbetalipoproteinemia, except in the homozygous familial form. 

Each class of lipoprotein has a specific function, determined by its point of synthesis, lipid composition, and apolipoprotein content. At least nine different apolipoproteins are found in the lipoproteins of human plasma (Table 21-3), distinguishable by their size, their reactions with specific antibodies, and their characteristic distribution in the lipoprotein classes. These protein components act as signals, targeting lipoproteins to specific tissues or activating enzymes that act on the lipoproteins. 

Keenan, T. W., Freudenstein, C. and Franke, W. W. 1977A. Membranes of mammary gland. XIII. A lipoprotein complex derived from bovine milk fat globule membrane with some preparative characteristics resembling those of actin. Cytobiologie 14, 259-278. 

It thus seemed that the origin of the various components in meat volatiles could best be established by analyzing irradiation-induced compounds in meat protein and meat fat separately. Accordingly, a 500-gram sample of meat, the same size of sample normally used in irradiation studies of whole meat, was separated into a protein, a lipid, and a lipoprotein fraction by means of a methanol-chloroform extraction of the fat. The dry, air-free, fractions were then irradiated separately with 6 megarads of gamma radiation in the manner used for whole meat. The analytical results (Table V) show clearly that mainly sulfur compounds and aromatic hydrocarbons are formed in the protein fraction, whereas mainly aliphatic hydrocarbons are formed from the lipid. The lipoprotein fraction produced, as expected, both aliphatic hydrocarbons and sulfur compounds. Only the lipoprotein fraction had a characteristic irradiation odor. 

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