Lipid composition of human serum lipoproteins

1. The lipid compositions of the low-density lipoproteins, the high-density lipoproteins and the ultracentrifugal residue of human serum are presented, with emphasis on certain lipoprotein classes and lipid components not previously described. 2. Except for the lipoproteins with the lowest and highest densities, there is a trend for stepwise successive increase or, respectively, decrease in the relative amounts of the main constituents of lipoproteins. 3. High-density lipoprotein-2 and high-density lipoprotein-3 have different amounts of certain lipids; high-density lipoprotein-2 has relatively more free cholesterol and sphingomyelin; high-density lipoprotein-3 has more free fatty acids, diglycerides and ceramide monohexosides. 4. All the lipoproteins contain hydrocarbons of the alkane series. The greatest amount, which averages 4.4% of total lipid extracted, is in the ultracentrifugal residue; n-alkanes comprise 18-50% of the hydrocarbons. 5. All the lipoproteins contain ceramide monohexosides. The highest relative contents of these glycolipids are in high-density lipoprotein-3 and in the ultracentrifugal residue. 6. The ultracentrifugal residue contains 55% of the total quantity of free fatty acids present in serum. The remaining free fatty acids are distributed among the other lipoprotein classes. 7. The choline-containing phospholipids (phosphatidylcholine, lysophosphatidylcholine and sphingomyelin) comprise about 90% of the phospholipids in all the lipoprotein classes except the low-density lipoprotein-2, which contains about 80% of these phospholipids. 8. The presence of a large amount of lysophosphatidylcholine in the ultracentrifugal residue and the successive decrease of sphingomyelin from the low-density lipoprotein-1 to the ultracentrifugal residue was confirmed. 9. The low-density lipoprotein-2 and the ultracentrifugal residue are characterized by relatively high contents of the lower glycerides.     

Amino acid composition and thermal stability of proteins

This study investigates the relationship between the thermal stability of a globular protein and its amino acid composition. The method deals with the relationship between the amino acid compositions and melting points in a set of proteins by computing single-residue and group correlations. Groups of residues are shown to stabilize or destabilize the molecule against temperature. The stabilizing group consists of polar-charged residues and nonpolar residues possessing high surrounding hydrophobicity. The polar-uncharged residues destabilize the molecule against temperature, serine being the most destabilizing residue. A very high cooperativity exists among the stabilizing nonpolar residues suggesting that their characteristic clustering inside the globule may enhance the thermostability of a protein. In small globular proteins which act as single cooperative units, the melting temperature remains mainly a function of amino acid composition, whereas in complex molecules it depends on other factors also.     

Amino acid composition of proteins extracted from virions of cytoplasmic-polyhedrosis virus

The ratio of basic to acidic amino acids in protein from free virions of CPV of Malacosoma disstria was 0.26, while that of occluded virions liberated from the polyhedra was 0.43. The ratio in free virions of CPV of Orgyia leucostigma was 0.39. This suggests that the virions of O. leucostigma reach a more advanced stage of maturity before occlusion than do those of M. disstria. By comparison, the composition of the free virions of O. leucostigma CPV was remarkably similar to that of the midgut protein of host cells. On the other hand, the composition of ribosomes of M. disstria was significantly different from either free or occluded virions of M. disstria CPV. Protein amino acids from virions of Bombyx mori differed from those of virions of other hosts in content of basic amino acids yielding a ratio of 0.68. These characteristics help to identify the viral strains and support previous serological studies reported with these proteins.     

Heat denaturation of human high density lipoproteins and chylomicrons

1. The objective of this investigation was to determine whether structural differences between apolipoproteins could be detected by heat denaturation. 2. The apoproteins of human serum high density lipoprotein (HDL2, d = 1.070-1.125 and HDL3, d = 1.125-1.21 g/ml), their major polypeptide constituents (R-Thr and R-Gln), and apochylomicrons were investigated. 3. Heat denaturation was found to be reversible in the temperature range from 20 to 80 degrees. 4. The thermodynamic parameters of heat denaturation delta F, delta H, delta S and delta Cp were calculated on the basis of a single transition from the "native" to "denatured" state for apo-HDL2, apochylomicrons, R-Thr and R-Gln; for apo-HDL3 these parameters were calculated on the basis of two transitions. 5. The thermodynamic parameters, with the exception of delta F, which describe heat denaturation of high density apolipoprotein, of high density apolipoprotein polypeptides and of apochylomicrons were found to be similar on a molar basis and to have approximately the same values as the thermodynamic parameters which describe heat denaturation of non-lipid binding proteins; on a weight basis differences were apparent between the apolipoproteins and the polypeptides or non-lipid binding proteins.     

Amino acid composition of proteins eluted from polyacrylamide gels: background considerations

It is shown that the contribution of the gel itself to amino acid compositions of proteins eluted from polyacrylamide gels is significant (especially at low protein-to-gel ratios), linearly related to the volume of the gel slices eluted, and, when applied as a correction to amino acid composition data, results in an enhancement of the composition determination.     

Amino acid sequence homologies between rabbit, rat, and human serum retinol-binding proteins

The main transporting protein for vitamin A in rabbit serum, the retinol-binding protein (RBP), was isolated and its amino acid sequence determined. Rabbit RBP was found to be highly homologous to human RBP, whose amino acid sequence was elucidated earlier, and to rat RBP. The rat RBP sequence was obtained by combining information deduced from the nucleotide sequences of two overlapping cDNA clones with the NH2-terminal sequence of the isolated protein determined by automated Edman degradation. The identity between the three proteins is approximately 90%. The high degree of homology between RBP molecules from different species is probably explained by the fact that RBP participates in at least three types of molecular interactions: in the binding of prealbumin, in the interaction with retinol, and in the recognition of a specific cell surface receptor. All these interactions should lead to a conservation of RBP structure. The amino acid differences between rabbit, rat, and human RBP are discussed in light of the recent elucidation of the three-dimensional structure of human RBP. Hybridization of a probe isolated from a rat RBP cDNA clone to restriction enzyme-digested genomic DNA from rat and mouse suggests that RBP is encoded by a single gene.     

Amino acid composition of human fibrinogen and anticoagulant derivatives

1. The amino acid compositions of human fibrinogen and three intermediate anticoagulant derivatives were determined by column chromatography. The derivatives were isolated by ammonium sulphate fractionation and column electrophoresis from solutions of fibrinogen undergoing spontaneous breakdown. One derivative, isolated as the large electrophoretic peak at the end of the clottable period (100% CP) of the parent fibrinogen solution, was labelled LP(100) and others obtained at twice this period (200% CP) were designated as LP(200) and SP(200) (LP, large peak; SP, small peak). 2. Maximal ;molecular' weights of approx. 294000 for LP(100), 137000 for LP(200) and 37000 for SP(200) were calculated for the protein moieties. At least 265 amino acid residues must have been lost from each fibrinogen molecule during the formation of LP(100), and 1362 during the formation of the other two derivatives. 3. Only one derivative (LP(200)) had a partial specific volume ([unk] 0.725ml./g.) different from that of fibrinogen ([unk] 0.721ml./g.). 4. No significant differences in refractive index at 589mmu were detected. 5. Calculation of the total number of ionizable groups/10(5)g. of each protein moiety showed a preponderance of the following numbers of negative charges: 22 in fibrinogen; 24 in LP(100); 26 in LP(200); 49 in SP(200). The isoionic points were estimated to be approx.+0.03pH unit (for fibrinogen), -0.06pH unit for (LP(100)) and +0.28pH unit (for LP(200)) from the pK of imidazole, and 0.78pH unit above the average pK of aspartyl and glutamyl ions (for SP(200)). These figures agree closely with experimentally determined values of the isoelectric point of fibrinogen and its derivatives.