Biomedical science最新文献

The physicochemical mechanism of aggregation of fibrinogen has been investigated in the presence and absence of fibrin-stabilising factor (factor XIIIa). Data from elastic and inelastic light-scattering and viscometry show that molecules of fibrinogen undergo a spontaneous modification of their carboxyl terminals and bind 'end to end' into flexible polymer chains. On attaining a critical length, the single-filament polymers twist into a coil and aggregate to form branched molecules in which the segments are packed sufficiently densely to resemble strongly hydrated globular particles. The formation, under the influence of factor XIIIa, of epsilon/gamma-glutamyl-lysine covalent bonds produces only insignificant changes in the spatial organisation of the fibrinogen aggregates. Covalent dimerisation of the gamma-chains restricts the structural flexibility of the polymers, but linking of the alpha-chains provides progressive compaction of the structure with increase in molecular weight. Electrophoresis of reconstituted samples shows that the coil-shaped chains of fibrinogen oligomers prevent the complete enzymatic linking of the gamma-chains. The results of this work suggest that the accelerated assembly of multimolecular aggregates, seen in the presence of factor XIIIa, may be explained by the stabilisation of intermediate complexes of fibrinogen, which makes the spontaneous transition from a stable native state to the activated state irreversible.

Murine tetrameric oxyhaemoglobin and insect monomeric erythrocruorin were studied. A doublet with the Lorentz form of lines (delta EQ = 2.22 mm s-1; delta alpha-Fe = 0.27 mm s-1; gamma 1/2 = 0.29 mm s-1) was observed in the oxyhaemoglobin spectrum at 4.2 K. In the 80-170 K interval the doublet components are distorted, but at T greater than 175 K, the lines again become symmetrical. In the 175-210 K region the value of gamma 1/2 is approximately 0.42 mm s-1. The profiles of the oxyhaemoglobin spectra are not dependent on the nature of the samples (blood; whole, or in aqueous or water-glycerol solutions at different pH values), or on the rate at which the latter are frozen. The oxyerythrocruorin spectra in aqueous solution and in a water-glycerol solution at 80 K and 170 K were doublets with Lorentz lines (the delta EQ values are equal to 2.20 mm s-1 and 2.12 mm s-1, respectively). It is concluded that the characteristics of the oxyhaemoglobin spectra reflect the specific electronic and structural properties of the oxy-complex in this protein. It was found that the oxyhaemoglobin spectra are very adequately described by two doublets with equal delta and gamma 1/2 values, but with different delta EQ values and relative intensities. A model is described in which these doublets correspond to two types of hydrogen bond associated with the distal histidine (E7), involving the terminal atom of molecular oxygen and the oxygen atom bound with the haem iron, respectively.

Purified preparations of bovine brain alpha-latrotoxin receptor contain proteins of 39 kDa and 65 kDa. Sequence analysis shows that the 65 kDa protein corresponds to p65, a synaptic vesicle membrane protein previously identified in rat synaptic vesicles, and that the 39 kDa protein is a proteolytic fragment of the 65 kDa protein. The 39 kDa and 65 kDa proteins appear in receptor samples because of their specific interaction with components of the alpha-latrotoxin receptor. This interaction may represent an essential step in perfusion and/or the fusion of synaptic vesicle membranes with the plasma membrane of the presynaptic terminal, both of which are final steps in the exocytosis of neurotransmitters.

A mathematical model of the response of cytotoxic T lymphocytes (CTL) to the nonexponential growth of an immunogenic tumour is presented. The model describes satisfactorily the kinetics of growth and regression of the B lymphoma BCL1 in the spleen of mice chimeric with respect to the major histocompatibility complex (H-2b----H-2d). Numerical estimations of the parameters describing processes that cannot be measured in vivo have been derived. It is predicted that the development of the tumour and its clinical manifestation have a recurrent profile with a cycle of three to four months. According to the model the limited growth of the BCL1 tumour and its transition into the dormant state are associated with a high rate of accumulation of CTL in the tumour, rapid lysis of lymphoma cells, and the inability of tumour cells to suppress the affector functions of cytotoxic lymphocytes. The model is also capable of representing the phenomenon of tumours 'sneaking through' and the immunostimulation of tumour growth.

The effect of intraperitoneal administration of phenobarbital (80 mg kg-1 body weight) and tryptophan (200 mg kg-1 body weight), separately or in combination, on the microsomal content of cytochrome P-450 and the activity of tryptophan 2,3-dioxygenase (EC 1.13.11.11) in Wistar rat liver was determined at different time intervals after injection. There was an increase in the amount of cytochrome P-450 within 12 h of administration of a single dose of phenobarbital which was maintained over the next 12 h. Tryptophan had no effect on the amount of cytochrome P-450, but administration of tryptophan in combination with phenobarbital blocked the increase that was found after administration of phenobarbital alone. Both phenobarbital and tryptophan increased tryptophan 2,3-dioxygenase activity (total enzyme and holoenzyme) but had different effects on the rate of activation and the degree of saturation of the enzyme with haem. Administration of tryptophan and phenobarbital in combination invoked the same effect as tryptophan alone. Significant activation of the holoenzyme was found, when tryptophan was administered 2 h after phenobarbital administration. It is proposed that combined administration of phenobarbital and tryptophan leads to substrate stabilisation of tryptophan 2,3-dioxygenase, and that this is accompanied by the binding of the newly synthesised haem, thus making haem unavailable for formation of cytochrome P-450.

Block copolymers of ethylene oxide and propylene oxide (pluronics) are nontoxic water-soluble membranotropic surfactants available as polymers with various compositions, molecular masses, number, and arrangement of blocks. In vivo experiments are reported which demonstrate that these polymers and their functional derivatives stimulate the production of anti-sheep-erythrocyte antibodies in mice. The introduction of reactive (hydroperoxide) groups into the polymers by chemical modification or by solubilization of low-molecular-mass hydroperoxides alters the properties of these immunostimulators. In vitro experiments revealed that these modified polymers enhance the activity of natural killer cells without reducing their viability. It is proposed that the immunomodulatory properties of pluronics and their derivatives play an important role in the antitumour activity of these substances in vivo.

The sequences of a 1.5 kb long stretch of the 5' flanking region of the gene for the alpha 3 isoform of the catalytic subunit of human Na(+)-K+ ATPase (located on chromosome 19) and of more than a 2 kb stretch of the 5' flanking region of the gene for the alpha 2 isoform (located on chromosome 1) have been determined. Transcription start sites for the gene for the alpha 3 isoform have been mapped at positions -152 and -155 relative to the translation initiation codon by primer extension analysis and S1-nuclease mapping of mRNA from human brain. The 5' flanking region of the gene for isoform alpha 3 contains a CCAAT box on the noncoding chain and six putative Sp1 binding sites. Absence of a conventional TATA box and a high GC content are other features of the region. The 5' upstream region of the gene for the alpha 2 isoform contains potential TATA and CCAAT boxes and one potential Sp1 binding site. Upstream of the putative TATA box there is an octanucleotide repeat, GGGGGAGA, which is also found in several eukaryotic genes in analogous positions. Pairwise comparison of the putative 5' regulatory regions of the genes coding for the different isoforms of the Na(+)-K(+)-ATPase catalytic subunit shows the existence of conserved elements, as well as of oligonucleotide blocks with very different structures. It is suggested that the differences in the primary structure of the 5' upstream regions may provide the basis for tissue-specific expression of the Na(+)-K(+)-ATPase isoforms.

The insertion of visual opsin into membranes occurred during in vitro translation of opsin mRNA in wheat germ extract in the presence of either microsomes or liposomes. The rhodopsin that integrated into both types of membranes after regeneration with 11-cis-retinal was functionally active (in contrast to the nonincorporated protein). Opsin either cotranslationally translocated into microsomes or inserted into liposomes had equal sensitivity to proteolysis and yielded the same pattern of peptides, which differed substantially from the set of peptides produced during proteolysis of opsin not incorporated into membranes. Thus visual opsin does not require protein translocation machinery for proper insertion into the lipid bilayer.