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The fermentation of xylose by Thermoanaerobacter ethanolicus ATCC 31938 was studied in pH-controlled batch and continuous cultures. In batch culture, a dependency of growth rate, product yield, and product distribution upon xylose concentration was observed. With 27 mM xylose media, an ethanol yield of 1.3 mol ethanol/mol xylose (78% of maximum theoretical yield) was typically obtained. With the same media, xylose-limited growth in continuous culture could be achieved with a volumetric productivity of 0.50 g ethanol/liter h and a yield of 0.42 g ethanol/g xylose (1.37 mol ethanol/mol xylose). With extended operation of the chemostat, variation in xylose uptake and a decline in ethanol yield was seen. Instability with respect to fermentation performance was attributed to a selection for mutant populations with different metabolic characteristics. Ethanol production in these T. ethanolicus systems was compared with xylose-to-ethanol conversions of other organisms. Relative to the other systems, T. ethanolicus offers the advantages of a high ethanol yield at low xylose concentrations in batch culture and of a rapid growth rate. Its disadvantages include a lower ethanol yield at higher xylose concentrations in batch culture and an instability of fermentation characteristics in continuous culture.  相似文献   
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A parametric finite element model of an osteocyte lacuna was developed to predict the microstructural response of the lacuna to imposed macroscopic strains. The model is composed of an osteocyte lacuna, a region of perilacunar tissue, canaliculi, and the surrounding bone tissue. A total of 45 different simulations were modeled with varying canalicular diameters, perilacunar tissue material moduli, and perilacunar tissue thicknesses. Maximum strain increased with a decrease in perilacunar tissue modulus and decreased with an increase in perilacunar tissue modulus, regardless of the thickness of the perilacunar region. An increase in the predicted maximum strain was observed with an increase in canalicular diameter from 0.362 to 0.421 microm. In response to the macroscopic application of strain, canalicular diameters increased 0.8% to over 1.0% depending on the perilacunar tissue modulus. Strain magnification factors of over 3 were predicted. However, varying the size of the perilacunar tissue region had no effect on the predicted perilacunar tissue strain. These results indicate that the application of average macroscopic strains similar to strain levels measured in vivo can result in significantly greater perilacunar tissue strains and canaliculi deformations. A decrease in the perilacunar tissue modulus amplifies the perilacunar tissue strain and canaliculi deformation while an increase in the local perilacunar tissue modulus attenuates this effect.  相似文献   
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Diatom systematics depends almost entirely upon structure of the silica shell. It is not known to what extent the taxonomic species, as defined by shell structure, corresponds to the genetic species—i.e., to the reproductively isolated population. As an approach to this problem, we report here a comparison of enzymes by electrophoresis. We have examined the genetic constitution of a number of clones of (presumably) the same species for each of 2 closely related, centric diatom species: Thalassiosira pseudonana Hasle and Heimdal and T. fluviatilis Hustedt. The 4 clones of T. fluviatilis form a distinct group, clearly separated from all the T. pseudonana clones. Within T. pseudonana, 4 estuarine clones and one reef clone form a group that is distinctly different from 4 oceanic clones. A single clone of T. pseudonana from the Continental Slope waters is intermediate between these 2 groups and probably shares genes with both groups, indicating that the 2 T. pseudonana groups are not genetically isolated. We conclude that i) within groups, isolates are closely related even though they originated from different continents; and, ii) T. pseudonana is subdivided into ecological races.  相似文献   
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This study aimed to determine the effect of supplementation with conjugated linoleic acids (CLAs) plus n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) on body composition, adiposity, and hormone levels in young and older, lean and obese men. Young (31.4+/-3.9 years) lean (BMI, 23.6+/-1.5 kg/m2; n=13) and obese (BMI, 32.4+/-1.9 kg/m2; n=12) and older (56.5+/-4.6 years) lean (BMI, 23.6+/-1.5 kg/m2; n=20) and obese (BMI, 32.0+/-1.6 kg/m2; n=14) men participated in a double-blind placebo-controlled, randomized crossover study. Subjects received either 6 g/day control fat or 3 g/day CLA (50:50 cis-9, trans-11:trans-10, cis-12) and 3 g/day n-3 LC-PUFA for 12 weeks with a 12-week wash-out period between crossovers. Body composition was assessed by dual-energy X-ray absorptiometry. Fasting adiponectin, leptin, glucose, and insulin concentrations were measured and insulin resistance estimated by homeostasis model assessment for insulin resistance (HOMA-IR). In the younger obese subjects, CLA plus n-3 LC-PUFA supplementation compared with control fat did not result in increased abdominal fat and raised both fat-free mass (2.4%) and adiponectin levels (12%). CLA plus n-3 LC-PUFA showed no significant effects on HOMA-IR in any group but did increase fasting glucose in older obese subjects. In summary, supplementation with CLA plus n-3 LC-PUFA prevents increased abdominal fat mass and raises fat-free mass and adiponectin levels in younger obese individuals without deleteriously affecting insulin sensitivity, whereas these parameters in young and older lean and older obese individuals were unaffected, apart from increased fasting glucose in older obese men.  相似文献   
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Telomere dysfunction results in fertility defects in a number of organisms. Although data from fission yeast and Caenorhabditis elegans suggests that telomere dysfunction manifests itself primarily as defects in proper meiotic chromosome segregation, it is unclear how mammalian telomere dysfunction results in germ cell death. To investigate the specific effects of telomere dysfunction on mammalian germ cell development, we examined the meiotic progression and germ cell apoptosis in late generation telomerase null mice. Our results indicate that chromosome asynapsis and missegregation are not the cause of infertility in mice with shortened telomeres. Rather, telomere dysfunction is recognized at the onset of meiosis, and cells with telomeric defects are removed from the germ cell precursor pool. This germ cell telomere surveillance may be an important mechanism to protect against the transmission of dysfunctional telomeres and chromosomal abnormalities.  相似文献   
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Transforming growth factor-β (TGF-β) stimulates the accumulation of extracellular matrix in renal and hepatic disease. Kidney glomerular mesangial cells (GMC) and liver fat-storing cells (FSC) produce latent or inactive TGF-β. In this study, we characterized the latent TGF-β complexes secreted by these cells. Human FSC produce a single latent TGF-β complex, predominantly of the TGF-β1 isoform, whereas GMC secrete multiple complexes of latent TGF-β, containing β1 and β2 isoforms. At least four forms were identified in GMC using ion exchange chromatography, including a peak not previously described in other cell types which eluted at 0.12 M NaCl, and predominantly of the β2 isoform. Both cell types secrete the latent TGF-β1 binding protein of 190 kDa, as part of a high molecular weight TGF-β complex. Epidermal growth factor stimulates the secretion of latent TGF-β and latent TGF-β binding protein in both cell types. Secretion of the latent TGF-β in both cell types was found to be associated with secretion of decorin. This study shows that vascular pericytes from the kidney and the liver have distinctly different profiles of latent TGF-β complexes, with GMC secreting a unique form of latent TGF-β2. The regulatory effect of epidermal growth factor and platelet-derived growth factor has potential implication for the pathophysiology of liver regeneration and chronic liver and kidney diseases. © 1996 Wiley-Liss, Inc.  相似文献   
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