Biphasic effect of fructose 2,6-bisphosphate on the liver fructose-1,6-bisphosphatase: mechanistic and physiological implications

Fructose 2,6-bisphosphate has been claimed to be both a substrate analogue and an allosteric inhibitor of fructose-1,6-bisphosphatase. The results reported here show that fructose 2,6-bisphosphate can be both an inhibitor and an activator of the enzyme, depending on the substrate concentration. This biphasic behaviour at saturating concentrations of substrate can only be due to an allosteric effect. In addition to the mechanistic implication it is possible that this finding may have physiological meaning.     

Differences in C-terminal amino acid sequences between erythrocyte and liver cytochrome b5 isolated from pig and human. Evidence for two tissue-specific forms of cytochrome b5

Two forms of cytochrome b5, a soluble erythrocyte form and a membrane-bound liver form, were purified from pig and human, and structural differences between them were analyzed. Porcine and human erythrocyte cytochrome b5 consisted of 97 amino acid residues and contained the same catalytic domain structure (residues 1-96) as that of the corresponding liver cytochrome b5, but had one amino acid replacement at the C-terminus (residue 97). These results suggest that erythrocyte cytochrome b5 is not derived from the liver protein by proteolysis but a translational product from another distinct mRNA of cytochrome b5.     

Structural relationship between the polyagglutinable antigen and the core polysaccharide of Pseudomonas aeruginosa

Abstract It has been observed that each strain of the Pseudomonas aeruginosa species harbours the so-called polyagglutinable antigen (PA). Some strains may produce it in a form which is linked to the core moiety of lipopolysaccharide (LPS) and this type of PA can thus be detected by passive haemagglutination using the isolated LPS as coating antigen. Other strains synthesize PA exclusively in a free form, which is also coextractable with LPS, its presence can, however, be demonstrated by the haemagglutination inhibition test. From a polyagglutinable strain of P. aeruginosa an R-type LPS was isolated having the core-linked PA. This LPS preparation was highly immunogenic with regard to its PA moiety. The core-bound PA seems to exert an immunosuppression on the core region, hence, the polyagglutinable strains isolated from cystic fibrosis patients only engender anti-PA antibodies, whereas antibodies against both, side chain and core region of LPS, are not engendered. The mucoid exopolysacharides also contains the PA which could possibly play an important role in the patient by protecting P. aeruginosa cells against anti-PA antibodies.     

High-performance liquid chromatographic quantitation of collagen biosynthesis in explant cultures

The capacity of lung explant cultures to synthesize collagen can be estimated by determining the content of [³H]hydroxyproline in protein following incubation with [³H]proline. The technique requires acid hydrolysis followed by quantitative separation of hydroxyproline from proline for scintillation counting and is often restricted to methods that can accommodate large samples because of relatively low specific radioactivity. A method which is useful for such samples, providing rapid separation of nonderivatized amino acids by ion-exchange HPLC, is described here. The HPLC system employs an HPX-87C cation-exchange column in 10 mm calcium acetate, pH 5.5, at 85°C. Under isocratic conditions hydroxyproline is completely resolved from proline with quantitative recovery of the ³H cpm applied to the column. Large amounts of material, equivalent to at least 150 mg wet wt of lung, can be applied without affecting resolution or recovery, and samples can be injected at intervals as short as 40 min. This method was used to study collagen biosynthesis in a model of pulmonary fibrosis induced in rabbits by the tumor-promoting agent, phorbol myristate acetate (PMA), and provides information concerning total protein synthesis as well as production of collagen. The data show a doubling in the rate of collagen production in lung explants prepared from animals treated with PMA compared with explants from control animals.     

硒的抗癌机制研究——亚硒酸钠对环磷鸟苷的作用

 AFB_1和NDEA与正常小鼠肝切片培养5min即可使cGMP分别升高一倍及一倍半(p<0.001);当有Na_2SeO_3(1μg/mL培养液)存在时,此作用可被消除,单独加入Na_2SeO_3对cGMP无影响。培养介质中存在磷酸二酯酶抑制剂茶碱时,除cGMP基础水平提高外,上述作用均相同。荷腹水型肝癌(HepA)小鼠腹水细胞中cGMP比正常肝中cGMP水平高三倍,连续4天腹腔给硒(Na_2SeO_3、1mg/kg体重)可使癌细胞中cGMP下降一倍。此剂量对正常小鼠肝中cGMP无影响;Na_2SeO_3(1μg/mL)与腹水细胞在体外短时间培养时,对细胞存活无影响,却可使cGMP含量下降。     

Heterogeneous distribution of nuclear triiodothyronine receptors in liver and preadipose cells as evidenced by their reactivity to antibodies against different protein products of erb A oncogenes

Polyclonal antibodies raised in rabbits against bacterially produced peptides in the C-terminal region of v-erb A or human c-erb A oncogenes recognize the nuclear triiodothyronine (T3) receptors in the T3-sensitive Ob 17 mouse preadipocyte cell line and not in mouse or rat liver. The results confirm the existence of different T3 receptors in different tissues. The results also suggest a heterogeneous receptor distribution within the preadipose cell line, with a predominance of c-erb A-type species. Antibodies raised against domain 149–227, but not against domain 245–325, impair T3 binding, suggesting a role for this domain in ligand binding.     

Activation of chloride conductance in pig jejunal brush border vesicles

Summary Requirements for the activation of Cl conductance have been investigated in pig jejunal brush border vesicles. The stability of ATP as a substrate for protein kinase activity, the stability of the phosphoprotein product of protein kinase action, and the choice of buffer system used for vesicle preparation were studied as variables which affected the outcome of in vitro activation attempts. Arsenate was selected as the most effective agent in protecting ATP from hydrolysis by the phosphatase activity in this vesicle system. Brush border vesicle protein appeared to prevent the accumulation of phosphoprotein in a cAMP-dependent protein kinase reaction, and vesicle protein only had phosphate acceptor activity when KF was added as a presumptive inhibitor of phosphoprotein phosphatase.A Cl conductance response to a potassium gradient and valinomycin was present in vesicles prepared in buffers containing tetramethylammonium. Cl^– conductance activity was not increased in this system by the addition of ATP, dibutyryl cyclic AMP, and cyclic AMP-dependent protein kinase.There was no Cl conductance response to a potassium gradient in vesicles buffered with imidazolium-acetate. Incorporation of ATP, AsO ₄ ^3– , and F^– into these nonconductive vesicles by homogenization, followed by addition of dibutyryl cAMP, produced substantial conductance activity. Maximal activation of Cl^– conductance was obtained with vesicles prepared in imidazolium-acetate buffering, using precautions to stabilize ATP and phosphoprotein prior to conductance measurements.     

DNA microarrays to define and search for genes associated with obesity

One of the major goals of this review was to identify obesity-specific gene profiles in animal models to help comprehend the pathogenic mechanisms and the prediction of the phenotypic outcomes of obesity and its associated metabolic diseases. The genomic examination of insulin-sensitive tissues, such as the adipose and hepatic tissues, has provided a wealth of information about the changes in gene expression in obesity and its associated metabolic diseases. The overexpression of genes related to inflammation, immune response, adhesion molecules, and lipid metabolism is a major characteristic of white adipose tissue, while the overexpression of the genes related to lipid metabolism, adipocyte differentiation, defense, and stress responses is noticeable in the non-alcoholic fatty liver of obese rodents. The hepatic-gene expression profiles led us to hypothesize that in obese rodents, the livers are supplied with large amounts of free fatty acids under conditions associated with obesity either through increased fatty acid biosynthesis or through decreased fatty acid oxidation, which may lead to increased mitochondrial respiratory activity. The wide list of genes that were identified in previous studies could be a source of potential therapeutic targets because most of these genes are involved in the key mechanisms of obesity development, from adipocyte differentiation to the disturbance of metabolism.     

Induction and Evasion of Innate Antiviral Responses by Hepatitis C Virus

Persistent hepatitis C virus infection is associated with progressive hepatic fibrosis and liver cancer. Acute infection evokes several distinct innate immune responses, but these are partially or completely countered by the virus. Hepatitis C virus proteins serve dual functions in replication and immune evasion, acting to disrupt cellular signaling pathways leading to interferon synthesis, subvert Jak-STAT signaling to limit expression of interferon-stimulated genes, and block antiviral activities of interferon-stimulated genes. The net effect is a multilayered evasion of innate immunity, which negatively influences the subsequent development of antigen-specific adaptive immunity, thereby contributing to virus persistence and resistance to therapy.