Cholic acid uptake and isolated rat hepatocytes.

Cholic acid uptake was studied in isolated rat hepatocytes using a centrifugal filtration technique to allow rapid sampling. Hepatocytes were found to adsorb as well as to transport cholic acid. The adsorption was characterized by a capacity of 24 nmol X mg cell protein-1 and an association constant of 0.59 X 103 M-1. Cholic acid uptake was linear with respect to concentration at or below 10 degree C, suggesting a unsaturable uptake process which was considered to represent simple diffusion and is quantitated by a diffusion coefficient of 1.76 pmol cholic acid X min-1 X mg protein-1 X muM-1. Above 10 degrees C the uptake curve was biphasic. After subtracting the unsaturable component from uptake rates at higher temperatures, a curve showing saturable kinetics resulted. The apparent Km and V values at 37 degrees C were calculated to be 31muM and 0.8 nmol X min-1 X mg protein-1 respectively. This saturable uptake process was temperature-dependent with an activation energy of 13 kcal X mol-1 (5.44 X 104 J X mol-1) and was inhibited by oligomycin and KCN. Countertransport was demonstrated with cholic, taurocholic and chenodeoxycholic acids. The results suggest that cholic acid is transported by an energy-dependent carrier-mediated process in addition to simple diffusion by hepatocytes, and that the postulated carrier has affinity for other bile acids.     

The lepidopteran mitochondrial control region: structure and evolution

For several species of lepidoptera, most of the approximately 350-bp mitochondrial control-region sequences were determined. Six of these species are in one genus, Jalmenus; are closely related; and are believed to have undergone recent rapid speciation. Recent speciation was supported by the observation of low interspecific sequence divergence. Thus, no useful phylogeny could be constructed for the genus. Despite a surprising conservation of control-region length, there was little conservation of primary sequences either among the three lepidopteran genera or between lepidoptera and Drosophila. Analysis of secondary structure indicated only one possible feature in common--inferred stem loops with higher-than-random folding energies-- although the positions of the structures in different species were unrelated to regions of primary sequence similarity. We suggest that the conserved, short length of control regions is related to the observed lack of heteroplasmy in lepidopteran mitochondrial genomes. In addition, determination of flanking sequences for one Jalmenus species indicated (i) only weak support for the available model of insect 12S rRNA structure and (ii) that tRNA translocation is a frequent event in the evolution of insect mitochondrial genomes.      

Glutamine repeats and inherited neurodegenerative diseases: molecular aspects

Several dominantly inherited, late onset, neurodegenerative diseases are due to expansion of CAG repeats, leading to expansion of glutamine repeats in the affected proteins. These proteins are of very different sizes and, with one exception, show no sequence homology to known proteins or to each other; their functions are unknown. In some, the glutamine repeat starts near the N-terminus, in another near the middle and in another near the C-terminus, but regardless of these differences, no disease has been observed in individuals with fewer than 37 repeats, and absence of disease has never been found in those with more than 41 repeats. Protein constructs with more than 41 repeats are toxic to E. coli and to CHO cells in culture, and they elicit ataxia in transgenic mice. These observations argue in favour of a distinct change of structure associated with elongation beyond 37–41 glutamine repeats. The review describes experiments designed to find out what these structures might be and how they could influence the properties of the proteins of which they form part. Poly- -glutamines form pleated sheets of β-strands held together by hydrogen bonds between their amides. Incorporation of glutamine repeats into a small protein of known structure made it associate irreversibly into oligomers. That association took place during the folding of the protein molecules and led to their becoming firmly interlocked by either strand- or domain-swapping. Thermodynamic considerations suggest that elongation of glutamine repeats beyond a certain length may lead to a phase change from random coils to hydrogen-bonded hairpins. Possible mechanisms of expansion of CAG repeats are discussed in the light of looped DNA model structures.     

Involvement of protein kinase A in the regulation of intracellular free calcium and phosphoinositide turnover in rat myometrium

Preincubation of Fura 2-loaded rat myometrial cells with H-8, an inhibitor of protein kinase A, for 1 h reversed the inhibitory effects of 8-(4-chlorophenylthio)-cAMP (CPTcAMP) on the oxytocin-stimulated increase in (Ca2+)i (intracellular free calcium), with an EC50 of 47 microM. H-8 also prevented the inhibition by relaxin and isoproterenol of the oxytocin-induced increase in (Ca2+)i. The EC50 of H-8 in reversing the relaxin effect was 42 microM. H-8 reversal of the effect of relaxin on (Ca2+)i was evident both in the absence of extracellular calcium and in cells pretreated with pertussis toxin. H-8 also reversed the inhibitory effects of relaxin and CPTcAMP on the oxytocin-induced increase in [3H]inositol phosphate formation and [3H]phosphoinositide hydrolysis. Preincubation of myometrial cells for 1 h with H-7, another protein kinase inhibitor, only partially attenuated the inhibition by relaxin and CPTcAMP of the oxytocin-induced increase in (Ca2+)i and [3H]inositol phosphate formation at concentrations 4-5 times greater than those of H-8. Acute (15-min) exposure to phorbol myristate acetate (1.0 microM) did not affect basal (Ca2+)i or the oxytocin-stimulated increases in (Ca2+)i or inositol phosphate formation. These results imply a regulatory role for protein kinase A in the inhibition of the oxytocin-induced increase in (Ca2+)i and inositol phosphate formation by relaxants.     

Development of cell surface saccharides on embryonic pancreatic cells

Using a battery of seven lectin-ferritin conjugates as probes for cell surface glycoconjugates, we have studied the pattern of plasmalemmal differentiation of cells in the embryonic rat pancreas from day 15 in utero to the early postpartum stage. Our results indicate that differentiation of plasmalemmal glycoconjugates on acinar, endocrine, and centroacinar cells is temporally correlated with development and is unique for each cell type, as indicated by lectin-ferritin binding. Specifically, (a) expression of adult cell surface saccharide phenotype can be detected on presumptive acinar cells as early as 15 d in utero, as indicated by soybean agglutinin binding, and precedes development of intracellular organelles characteristic of mature acinar cells; (b) maturation of the plasmalemma of acinar cells is reached after intracellular cytodifferentiation is completed, as indicated by appearance of Con A and fucoselectin binding sites only at day 19 of development; conversely, maturation of the endocrine cell plasmalemma is accompanied by "loss" (masking) of ricinus communis II agglutinin receptors; and (c) binding sites for fucose lectins and for soybean agglutinin are absent on endocrine and centroacinar cells at all stages examined. We conclude that acinar, centroacinar, and endocrine cells develop from a common progenitor cell(s) whose plasmalemmal carbohydrate composition resembles most closely that of the adult centroacinar cell. Finally, appearance of acinar lumina beginning at approximately 17 d in utero is accompanied by differenetiation of apical and basolateral plasmalemmal domains of epithelial cells, as indicated by enhanced binding of several lectin-ferritin conjugates to the apical plasmalemmal, a pattern that persists from this stage through adult life.     

Characterization of antifungal metabolites produced by Lactobacillus plantarum and Lactobacillus coryniformis isolated from rice rinsed water

Molecular Biology Reports - A recent spike in demand for chemical preservative free food has derived the scientific community to develop natural ways of food preservation. Therefore,...     

Identification and characterization of functional intermediates of stem bromelain during urea and guanidine hydrochloride unfolding

By comparing changes in enzyme activity with changes in spectral features for stem bromelain (EC.3.4.22.32) in the absence and presence of urea, Guanidine hydrochloride and ethanol; four intermediate states could be identified: two activity-enhanced state obtained in the presence of 5 M urea and 2 M GnHCl, termed X and X', respectively, and a third, similarly active state closely resembling the native protein in the presence of 8-9 M urea, termed Y. The enhanced activity of these states is due to local conformational changes accompanied by increased dynamics in the active site. Further, the enzyme does not lose its activity after substantial tertiary structure changes in 8-9 M urea (Y state), suggesting that active site containing domain is more resistant to chemical denaturation than the other structural domain. This makes stem bromelain and in general cysteine proteases an exception to the hypothesis that active site is the most labile part of enzyme.     

Activation of cAMP-guanine exchange factor confers PKA-independent protection from hepatocyte apoptosis

cAMP has previously been shown to promote cell survival in a variety of cell types, but the downstream signaling pathway(s) of this antiapoptotic effect is unclear. Thus the role of cAMP signaling through PKA and cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs) in cAMP's antiapoptotic action was investigated in the present study. cAMP's protective effect against bile acid-, Fas ligand-, and TNF-alpha-induced apoptosis in rat hepatocytes was largely unaffected by the selective PKA inhibitor, Rp-8-(4-chlorophenylthio)-cAMP (Rp-cAMP). In contrast, a novel cAMP analog, 8-(4-chlorophenylthio)-2'-O-methyl (CPT-2-Me)-cAMP, which activated cAMP-GEFs in hepatocytes without activating PKA, protected hepatocytes against apoptosis induced by bile acids, Fas ligand, and TNF-alpha. The role of cAMP-GEF and PKA on activation of Akt, a kinase implicated in cAMP survival signaling, was investigated. Inhibition of PKA with RP-cAMP had no effect on cAMP-mediated Akt phosphorylation, whereas CPT-2-Me-cAMP, which did not activate PKA, induced phosphatidylinositol 3-kinase (PI3-kinase)-dependent activation of Akt. Pretreatment of hepatocytes with the PI3-kinase inhibitor, Ly-294002, prevented CPT-2-Me-cAMP's protective effect against bile acid and Fas ligand, but not TNF-alpha-mediated apoptosis. Glucagon, CPT-cAMP, and CPT-2-Me-cAMP all activated Rap 1, a downstream effector of cAMP-GEF. These results suggest that a PKA-independent cAMP/cAMP-GEF/Rap pathway exists in hepatocytes and that activation of cAMP-GEFs promotes Akt phosphorylation and hepatocyte survival. Thus a cAMP/cAMP-GEF/Rap/PI3-kinase/Akt signaling pathway may confer protection against bile acid- and Fas-induced apoptosis in hepatocytes.