Conformation of apolipoprotein B-100 in the low density lipoproteins of tangier disease. Identification of localized conformational response to triglyceride content

The low density lipoproteins (LDL) from patients with Tangier disease are enriched in triglycerides, 27% of LDL mass versus 7% for normal LDL. To study whether this unique LDL core lipid composition affects the surface disposition of apolipoprotein (apo) B-100, we analyzed the LDL by protease digestion and in competitive radioimmunoassays. Limited proteolytic digestion of Tangier LDL by Staphylococcus aureus V8 protease generated a prominent fragment of 120 kDa (cleavage site at residue 1076), which was not visible in similarly digested normal LDL. In competitive radioimmunoassay, Tangier LDL bound weakly to the apoB-specific monoclonal antibody MB20, compared with control LDL. We localized the MB20 epitope between residues 1031 and 1084 of apoB-100, probably very near residue 1076. DNA sequencing of exon 21 of apoB genomic clones (coding for residues 1014-1084) from a Tangier patient revealed no difference from the normal DNA sequence, thus eliminating a protein polymorphism as a basis for the altered protease sensitivity and antibody binding. When the triglyceride contents of Tangier LDL were reduced to 10% of mass by incubation with normal high density lipoproteins, production of the 120-kDa fragment by proteolysis decreased and MB20 binding increased in affinity, implying a change toward normal conformation of apoB-100. Thus, using two independent techniques, proteolytic digestion and binding of monoclonal antibodies, we have demonstrated an alternative conformation of apoB-100 in the vicinity of residue 1076, which reflects the content of triglycerides in the LDL particle.

Singlet-singlet energy transfer along the helix of a double-stranded nucleic acid at room temperature

An irreversible electronic energy trap has been formed in calf thymus DNA by methylating about 75% of its G bases at position N-7. This has allowed us to measure for the first time the efficiency of transfer of energy along the helix of a double-stranded nucleic acid at room temperature. It is found that about one out of every three photons absorbed by the other bases is trapped. We have also simulated the data with a stochastic model that uses the dipole-dipole interaction to calculate the efficiency of transfer. In order to approximate the experimental results, the model requires that: (i) the fluorescence quantum yield of T, C, and G in DNA be about 2 x 10(-3), which is about two orders of magnitude larger than the value of the fluorescence quantum yield reported for DNA; and (ii) the fluorescence quantum yield of A in DNA be negligibly small. Requirement (i) is consistent with energy transfer taking place before a very efficient fluorescence quenching process sets in, which could be formation of excited-state complexes (excimers) that do not fluoresce appreciably. Requirement (ii) implies a very short fluorescence lifetime for A, which is consistent with the reported absence of a significant number of photoproducts formed by A in DNA. The simulations find that, on the average, the excitation energy takes about 1.2 steps to reach the trap; that is to say, bases that are nearest and next nearest neighbors of the trap are, in effect, the only energy donors. Both intra- as well as interstrand energy transfer (the latter only for the C-trap base pair) make significant contributions. The value of the efficiency for pairwise base-base intrastrand transfer is about 60%, whereas those for base-trap intra- and interstand transfer are 90% and 80%, respectively. The corresponding values for the rate constant of transfer are 2 x 10(11), 1 x 10(12), and 4 x 10(11) s-1. Transfer is inefficient when A is the donor or the acceptor. In addition to the dipole-dipole term, the only other significant term in the expansion of the interaction potential is the dipole-quadrupole term which, however, makes only a small contribution to the overall transfer efficiency. The electron exchange interaction appears to be much less efficient than the coulombic interaction.

Fluorometric analysis of the long-wavelength absorption band of N-7 methylated GMP into the constituent bands of the two electronic states

Steady-state measurements of fluorescence anisotropy are used to resolve the long-wavelength absorption spectrum of 7-methyl guanosine 5'-monophosphate (GMP) in pH 5 buffer at room temperature into component spectra that correspond to the electronic transitions I and II present in that spectral region. We have chosen this derivative of guanine because its fluorescence quantum yield is much greater than that of GMP. It is found that the data are adequately described by a model that involves emission exclusively from state I, with state II converting to it with 100% efficiency. The shape of the absorption spectrum of state II is virtually independent of the angle theta between the absorption transition dipole moments of states I and II, whereas that of state I is dependent on theta. We analyze the data on the basis of the premise that in the short-wavelength region state II is the predominantly absorbing state. This premise is based on studies of single-crystal polarized reflection and linear dichroism from stretched films. The spectral maxima for the two states are found to be at about 290 and 260 nm, respectively. There is also a weak band which is centered at about 245 nm. The oscillator strengths are found to be 0.07, 0.21 and approximately 0.04, for states I, II and that associated with the weak band, respectively. The importance of these findings with regard to the photophysical properties of nucleic acids and calculations of their CD spectra is discussed.

[Application of ion chromatography to identification of anaerobic bacteria]

The fatty acids of anaerobic bacteria were detected by ion chromatography (IC). The results showed that 11 kinds of standard fatty acids could be analyzed within 15 min, and just needed 1 injection sample. The fatty acids patterns of 40 strains of anaerobic bacteria detected by IC were essentially the same as that by gas chromatography (GC) reported by VPI. In comparison with GC, IC has further advantages: pretreatment is simpler, both volatile and non-volatile fatty acids can be analyzed at the same time, and formic and lactic acid can be well analyzed.

Birefringent filter with tilted optic axis for tuning dye lasers: theory and design

The dependence of the tuning behavior of a birefringent filter on the orientation of the optic axis to the surface of the plate is studied. The Jones matrix representing a tilted birefringent plate is reduced to practical form, and this leads to simplifying the theoretical operation of tuning a birefringent filter in a laser resonator. The principles of proper determination of the filter parameters are given. A design method, which takes into consideration the bandwidth, rejection and tuning range of the filter, and design examples, is provided.

Light and electron microscopic evidences of the presence of c-fos-like immunoreactivity in the rat adrenal cortex

The presence and localization of c-fos-like immunoreactivity in the rat adrenal cortex has been demonstrated by immunocytochemical methods at both light and electron microscopic level. C-fos-like immunoreactivity was detected in the zona fasciculata and the zona reticulata, but not in the zona glomerulosa. Ultrastructurally, all products of c-fos-like immunoreaction were localized exclusively in the regions associated with the euchromatin in the nucleus of the immunoreactive cells. Moreover, a higher density of the immunoreactive cells in the adrenal cortex of pregnant rats was found with quantitative immunocytochemistry as compared to the non-pregnant. The characteristic zonation of c-fos-like immunoreactivity in the adrenal cortex suggest that the c-fos protein is involved in the normal function of the glucocorticoid-producing cells of mammalian adrenals. The numerical increase in the immunoreactive cells in pregnant rats implies that basal expression of the c-fos-like protein may vary with the functional state of the cortical cells.

Relationship of microglia and astrocytes to amyloid deposits of Alzheimer disease

The relationship of microglia and astrocytes to deposits of beta-amyloid protein (BAP) was studied in Alzheimer's disease by immunohistochemistry. BAP was detected in forms varying from diffuse amorphous deposits to compact spherical masses. These latter corresponded in frequency and distribution to senile plaques revealed by Bielschowsky and thioflavine S staining. Approximately half the diffuse deposits had no human leukocyte antigen (HLA)-DR-positive reactive microglia associated with them while nearly all compact deposits had single or multiple HLA-DR-positive reactive microglia embedded in their core. Electron microscopy showed these reactive microglia to be in intimate contact with amyloid fibrils. These data suggest that amyloid deposition may precede the activation of microglia.

Structural domains of human apolipoprotein B-100. Differential accessibility to limited proteolysis of B-100 in low density and very low density lipoproteins

The structural domains of human apolipoprotein B-100 in low density lipoproteins (LDL) and the conformational changes of B-100 that accompany the conversion of very low density lipoproteins (VLDL) to LDL were investigated by limited proteolysis with 12 endoproteases of various specificities, and their cleavage sites were determined. In B-100 of LDL, we identified two peptide regions that are highly susceptible to proteolytic cleavage. One region encompassed about 40 amino acids (residues 1280-1320, designated as the NH2-terminal region) and the other about 100 amino acids (residues 3180-3280, designated as the COOH-terminal region). IN LDL, the cleavage sites in both susceptible regions of B-100 were readily accessible to limited proteolysis; but in VLDL, only sites in the COOH-terminal region were readily accessible. Moreover, B-100 in VLDL appeared less degraded than B-100 in LDL by all enzymes used. Reduction of disulfide bonds of B-100 in both LDL and VLDL before digestion by Staphylococcus aureus V8 protease and clostripain exposed additional cleavage sites and increased the rate of B-100 degradation, suggesting that disulfide bonds probably exert conformational constraints. These results indicate the presence of three principal structural domains in B-100 of LDL that are relatively resistant to limited proteolysis. These three domains are connected by the two susceptible peptide regions. Our results also demonstrate differential accessibility of cleavage sites in B-100 of LDL and VLDL to limited proteolysis. This differential accessibility suggests that substantial changes in the conformation or environment of B-100 accompany the conversion of VLDL to LDL.

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.