Structural comparison of human apolipoproteins B-48 and B-100

Potential biomarkers in psychiatry: focus on the cholesterol system

Measuring biomarkers to identify and assess illness is a strategy growing in popularity and relevance. Although already in clinical use for treating and predicting cancer, no biological measurement is used clinically for any psychiatric disorder. Biomarkers could predict the course of a medical problem, and aid in determining how and when to treat. Several studies have indicated that of candidate psychiatric biomarkers detected using proteomic techniques, cholesterol and associated proteins, specifically apolipoproteins (Apos), may be of interest. Cholesterol is necessary for brain development and its synthesis continues at a lower rate in the adult brain. Apos are the protein component of lipoproteins responsible for lipid transport. There is extensive evidence that the levels of cholesterol and Apos may be disturbed in psychiatric disorders, including autistic spectrum disorders (ASD). Here, we describe putative serum biomarkers for psychiatric disorders, and the role of cholesterol and Apos in central nervous system (CNS) disorders.

Mapping oxidations of apolipoprotein B-100 in human low-density lipoprotein by liquid chromatography-tandem mass spectrometry

Human low-density lipoprotein (LDL) is a major cholesterol carrier in blood. Elevated concentration of low-density lipoprotein, especially when oxidized, is a risk factor for atherosclerosis and other cardiac inflammatory diseases. Past research has connected free radical initiated oxidations of LDL with the formation of atherosclerotic lesions and plaque in the arterial wall. The role of LDL protein in the associated diseases is still poorly understood, partially due to a lack of structural information. In this study, LDL was oxidized by hydroxyl radical. The oxidized protein was then delipidated and subjected to trypsin digestion. Peptides derived from trypsin digestion were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Identification of modified peptide sequences was achieved by a database search against apo B-100 protein sequences using the SEQUEST algorithm. At different hydroxyl radical concentrations, oxidation products of tyrosine, tryptophan, phenylalanine, proline, and lysine were identified. Oxidized amino acid residues are likely located on the exterior of the LDL particle in contact with the aqueous environment or directly bound to the free radical permeable lipid layer. These modifications provided insight for understanding the native conformation of apo B-100 in LDL particles. The presence of some natural variants at the protein level was also confirmed in our study.

Molecular and metabolic basis for the metabolic disorder normotriglyceridemic abetalipoproteinemia

We have previously described a disorder, normotriglyceridemic abetalipoproteinemia, that is characterized by the virtual absence of plasma low density lipoproteins and complete absence of apoB-100, but with apparently normal secretion of triglyceride-rich lipoproteins containing apoB-48. The patient's plasma lipoproteins were shown on polyacrylamide gels and by antibody mapping to have a new truncated apoB variant, apoB-50, circulating along with her apoB-48. We have found this individual to be homozygous for a single C-to-T nucleotide substitution at apoB codon 2252, which produces a premature in-frame stop codon. Thus, this is a rare example of homozygous hypobetalipoproteinemia. Electron photomicrographs revealed that the diameters of particles in the d less than 1.006 g/ml lipoprotein fraction, in both the postprandial and postabsorptive state, are bimodally distributed. The molar ratio of apoE to apoB in these particles is 3.5:1, similar to normal VLDL. The plasma LDL interval contains both spherical and cuboidal particles. Autologous reinfusion of labeled d less than 1.006 g/ml lipoproteins showed exponential disappearance from plasma, with an apparent half-removal time of 50 min, somewhat slower than for normal chylomicrons but within the normal range for VLDL. The calculated production rate for apoB was within the normal range in this subject. A very small amount of label was found briefly in the IDL fraction, but none at any time in LDL or HDL. Therefore, because LDL particles that contain apoB-50 lack the putative ligand domain of the LDL receptor, we conclude that the very low level of LDL is due to the rapid removal of the abnormal VLDL particles before their conversion to LDL can take place.

Separation and visualization of apolipoprotein B species by sodium dodecyl sulfate-agarose gel electrophoresis and immunoblotting

A method for separation and visualization of the different apolipoprotein B species using 0.2% sodium dodecyl sulfate-1.5% agarose gel electrophoresis and immunoblotting is described. The method is capable of demonstrating the different forms of apolipoprotein B (apo B) in plasma volumes of 10-50 microliters without prior ultracentrifugation. After ultracentrifugation of samples, estimation of the ratio between apo B 48 and apo B 100 is possible by scanning of Coomassie-stained gels or immunoblots.

Distribution of lipid-binding regions in human apolipoprotein B-100

The distribution of lipid-binding regions of human apolipoprotein B-100 has been investigated by recombining proteolytic fragments of B-100 with lipids and characterizing the lipid-bound fragments by peptide mapping, amino acid sequencing, and immunoblotting. Fragments of B-100 were generated by digestion of low-density lipoproteins (LDL) in the presence of sodium decyl sulfate with either Staphylococcus aureus V8 protease, pancreatic elastase, or chymotrypsin. Particles with electron microscopic appearance of native lipoproteins formed spontaneously when detergent was removed by dialysis from enzyme digests containing fragments of B-100 and endogenous lipids, or from incubation mixtures of delipidated B-100 fragments mixed with microemulsions of exogenous lipids (cholesteryl oleate and egg phosphatidylcholine). Fractionation of the recombinant particles by isopycnic or density gradient ultracentrifugation yielded complexes similar to native LDL with respect to shape, diameter, electrophoretic mobility, and surface and core compositions. Circular dichroic spectra of these particles showed helicity similar to LDL but a somewhat decreased content of beta-structure. Most of the fragments of B-100 were capable of binding to lipids; 12 were identified by direct sequence analysis and 14 by reaction with antisera against specific sequences within B-100. Our results indicate that lipid-binding regions of B-100 are widely distributed within the protein molecule and that proteolytic fragments derived from B-100 can reassociate in vitro with lipids to form LDL-like particles.

Structure of apolipoprotein B-100 of human low density lipoproteins

We have analyzed low density lipoproteins (LDL) apolipoprotein (apop) B structure by direct sequence analysis of LDL apo B-100 tryptic peptides. Native LDL were digested with trypsin, and the products were fractionated on a Sephadex G-50 column. The partially digested apo B-100 still associated with lipids was recovered in the void volume (designated trypsin-nonreleasable, TN, peptides). The released peptides (designated trypsin-releasable, TR, peptides) in subsequent peaks were repurified on two successive high-performance liquid chromatography (HPLC) columns. The TN peak was delipidated and redigested with trypsin, and the resulting peptides were purified on two successive HPLC columns. Using this approach, we sequenced over 88% of LDL apo B-100, extending and refining our previous study (Nature 1986;323:738-742) which covered 52% of the protein. TN peptides made up 31%, and the TR peptides, 34% of the apo B-100 sequence; 23.7% were found under both TN and TR categories. Based on its differential trypsin releasability, apo B-100 can be divided into five domains: 1) residues 1----1000, largely TR; 2) residues 1001----1700, alternating TR and TN; 3) residues 1701----3070, largely TN; 4) residues 3071----4100, mainly TR and mixed; and 5) residues 4101----4536, almost exclusively TN. Domain 1 contained 14 of the 25 Cys residues in apo B. Domain 4 encompassed seven N-glycosylation sites, and contained the putative receptor binding domains. All 19 potential N-glycosylation sites were directly sequenced: 16 were found to be glycosylated and three were not. Three pairs of disulfide bridges were also mapped. Finally, a combination of cDNA sequencing, direct mRNA sequencing, and comparison of published apo B-100 sequences allowed us to identify specific amino acid residues within apo B-100 that seem to represent bona fide allelic variations. Our study provides information on LDL apo B-100 structure that will be important to our understanding of its conformation and metabolism.

Hepatocytic lipoprotein receptors and intracellular lipoprotein catabolism

Hepatocytes, as the major site of synthesis and terminal catabolism of plasma lipoproteins, exert the major regulatory influence on the concentration of atherogenic lipoproteins in blood plasma and may thereby influence the rate of atherogenesis. The LDL receptor on the microvillous sinusoidal surface of hepatocytes mediates the catabolism of remnants of triglyceride-rich lipoproteins and LDL. Binding of VLDL remnants to the receptor, mediated by apo E, is of very high affinity and presumably multivalent, whereas binding of LDL, mediated by apo B-100, is monovalent and of lower affinity, accounting for the much longer residence time of the latter in the blood. The magnitude of the influx of lipoprotein particles into hepatocytic endosomal compartments dwarfs that of other macromolecules undergoing receptor-mediated endocytosis and terminal catabolism in lysosomes of these cells. The intracellular compartments and processing steps in hepatocytic lipoprotein uptake and degradation are essentially the same as those described for other ligands in the liver and other cells. Receptors with bound lipoproteins migrate into coated pits which become coated vesicles. These vesicles uncoat and fuse to form CURL vesicles and tubules near the cell surface where most receptors are recycled, presumably via receptor-rich appendages that become separated from the vesicles. CURL vesicles become mature MVBs as they migrate to the Golgi/bile canalicular pole of hepatocytes, where they fuse with putative Golgi-derived primary lysosomes and are transformed into heterophagic secondary lysosomes. MVBs also contain a receptor-rich appendage that may recycle some receptors directly to the cell surface or through adjacent Golgi compartments. Dilated ends of trans-Golgi cisternae contain nascent VLDL undergoing packaging for secretion following their synthesis and assembly in the endoplasmic reticulum. Because these "forming secretory vesicles" resemble remnant-filled MVBs, occur in a similar location in the Golgi area of hepatocytes and coisolate in centrifugal fractions of liver homogenates, there has been considerable confusion about the identity of these compartments. With the aid of specific endocytic and exocytic markers, highly purified and morphologically intact endosomal and Golgi compartments can now be obtained from rat liver homogenates. The availability of these and similar fractions of defined purity should facilitate investigation of the hepatocytic processing of endocytosed and secreted macromolecules. Although chylomicron remnants are also taken up by receptor-mediated endocytosis, the nature of the hepatocytic remnant receptor remains elusive.(ABSTRACT TRUNCATED AT 400 WORDS)

Degradation of apolipoprotein B-100 in human chylomicrons

The purpose of this study was to investigate the molecular forms of apolipoprotein B (ApoB) in human chylomicrons under well-preserved conditions. To this end, plasma and serum were collected from the same normal subjects after ingestion of a fatty meal. The samples were divided into three or four aliquots before the addition of various preservative mixtures, including antibiotics, antioxidants and proteinase inhibitors. The chylomicrons were isolated immediately, and all steps were carried out at or below 4 degrees C. Changes in the molecular weight of ApoB in chylomicrons were followed by a time study using 3.3% polyacrylamide gel electrophoresis containing SDS. ApoB from chylomicrons analyzed within 5 h of blood collection showed a single band with mobility identical to that of ApoB (ApoB-100) in low-density lipoproteins. When analyzed after 1-2 days, satellite bands smaller than ApoB-100 were observed, and a very faint band with Mr 200,000 appeared, which comigrated with intestinal ApoB (ApoB-48). Upon storage, the molecular weight of ApoB was smaller in chylomicrons subjected to a higher number of reflotations than those in chylomicrons washed less frequently, suggesting that purified chylomicrons degrade faster. A longer storage time at 4 degrees C (i.e., 7 or 14 days) revealed a stepwise degradation of ApoB, yielding Mr 200,000 band as the prominent form. The degradation of ApoB-100 was slower when both proteinase inhibitors, leupeptin and epsilon-amino caproic acid, were employed, and the appearance of Mr 200,000 band was quicker when the chylomicrons were processed at higher temperature (15-25 degrees C) in the absence of a proteinase inhibitor. Immunoblotting shows that the segment removed from ApoB-100 was the carboxyl-terminal portion. These results suggest the possible presence of a proteinase(s), which copurified with chylomicrons, and which converts ApoB-100 from a large to a smaller molecular form. Although the stop codon has been discovered recently in intestinal ApoB mRNA, which explains the mechanism for direct synthesis of ApoB-48, apparently ApoB-100 is also synthesized in the intestine of all eight subjects studied here, and the ApoB-100 degrades to a form which is ApoB-48-like.

Carboxyl terminal analysis of human B-48 protein confirms the novel mechanism proposed for chain termination

In this study we have confirmed the presence of a single base difference between intestinal mRNA coding for B-48 and hepatic mRNA coding for B-100, which results in the substitution of a stop codon (UAA) for a glutamine codon (CAA) at a point corresponding to amino acid residue 2153 in the B-100 sequence. Based on this finding, B-48 is predicted to terminate at residue 2152 with the sequence ... Met Ile. To confirm this finding at the protein level, B-48 and B-100 were each digested with cyanogen bromide and the digestion products were analysed for the presence of isoleucine. Isoleucine was found only in cyanogen bromide digests of B-48 confirming that only B-48 terminates with the predicted amino acid sequence ... Met Ile.