排序方式: 共有55条查询结果,搜索用时 0 毫秒
1.
Stephen Bornemann David H. G. Crout Howard Dalton David W. Hutchinson 《Biocatalysis and Biotransformation》1994,11(3):191-221
The effect of surfactants on the hydrolysis of prochiral and chiral substrates by crude and purified porcine pancreatic lipase (PPL, EC 3.1.1.3)) has been studied. Rather than accelerating the reactions, surfactants slowed down (“inhibited”) the reactions relative to the rate in the absence of surfactant. Surfactants varied in the extent to which the reaction was inhibited. With the crude enzyme there was a correlation between degree of inhibition and the optical purity of the product of hydrolysis of an achiral diester substrate 1. There was no special effect associated with use of surfactants in the concentration range corresponding to critical micelle formation, nor was there any increase in rate of reaction when stable emulsions were formed by using mixtures of surfactants to generate an appropriate hydrophile-lipophile (HLB) balance. A study of the effect of sodium dodecyl sulphate (SDS) on the hydrolysis of the diester 1 by crude PPL showed that the rate of the reaction steadily decreased with increasing surfactant concentration, but that the optical purity of the product first fell and then rose gain, an effect attributed to the differential denaturing action of the surfactant on at least three hydrolytic enzymes. In general, there would seem to be no advantage to be gained from the use of surfactants in the hydrolysis by PPL of compounds of low water solubility; the use of an immiscible co-solvent is more effective. 相似文献
2.
3.
Galeriu D Crout NM Melintescu A Beresford NA Peterson SR Van Hees M 《Radiation and environmental biophysics》2001,40(4):325-334
Tritium is a potentially important environmental contaminant originating from the nuclear industry, and its behaviour in the
environment is controlled by that of hydrogen. Animal food products represent a potentially important source of tritium in
the human diet and a number of transfer coefficient values for tritium transfer to a limited number of animal products are
available. In this paper we present an approach for the derivation of tritium transfer coefficients which is based on the
metabolism of hydrogen in animals. The derived transfer coefficients separately account for transfer to and from free (i.e.
water) and organically bound tritium. A novel aspect of the approach is that tritium transfer can be predicted for any animal
product for which the required metabolic input parameters are available. The predicted transfer coefficients are compared
to available independent data. Agreement is good (R
2=0.97) with the exception of the transfer coefficient for transfer from tritiated water to organically bound tritium in ruminants.
This may be attributable to the particular characteristics of ruminant digestion. We show that tritium transfer coefficients
will vary in response to the metabolic status of an animal (e.g. stage of lactation, diet digestibility etc.) and that the
use of a single transfer coefficient from diet to animal product is inappropriate. It is possible to derive concentration
ratio values from the estimated transfer coefficients which relate the concentration of tritiated water and organically bound
tritium in an animal product to their respective concentrations in the animals diet. These concentration ratios are shown
to be less subject to metabolic variation and may be more useful radioecological parameters than transfer coefficients. For
tritiated water the concentration ratio shows little variation between animal products ranging from 0.59 to 0.82. In the case
of organically bound tritium the concentration ratios vary between animal products from 0.15 (goat milk) to 0.67 (eggs).
Received: 28 May 2001 / Accepted: 20 August 2001 相似文献
4.
Steve Horvath Abu NM Nazmul-Hossain Rodney PE Pollard Frans GM Kroese Arjan Vissink Cees GM Kallenberg Fred KL Spijkervet Hendrika Bootsma Sara A Michie Sven U Gorr Ammon B Peck Chaochao Cai Hui Zhou David TW Wong 《Arthritis research & therapy》2012,14(6):1-13
Bone tissue has an exceptional quality to regenerate to native tissue in response to injury. However, the fracture repair process requires mechanical stability or a viable biological microenvironment or both to ensure successful healing to native tissue. An improved understanding of the molecular and cellular events that occur during bone repair and remodeling has led to the development of biologic agents that can augment the biological microenvironment and enhance bone repair. Orthobiologics, including stem cells, osteoinductive growth factors, osteoconductive matrices, and anabolic agents, are available clinically for accelerating fracture repair and treatment of compromised bone repair situations like delayed unions and nonunions. Preclinical and clinical studies using biologic agents like recombinant bone morphogenetic proteins have demonstrated an efficacy similar or better than that of autologous bone graft in acute fracture healing. A lack of standardized outcome measures for comparison of biologic agents in clinical fracture repair trials, frequent off-label use, and a limited understanding of the biological activity of these agents at the bone repair site have limited their efficacy in clinical applications. 相似文献
5.
Stereochemistry of the incorporation of valine methyl groups into methylene groups in cephalosporin C. 下载免费PDF全文
C P Pang R L White E P Abraham D H Crout M Lutstorf P J Morgan A E Derome 《The Biochemical journal》1984,222(3):777-788
'Chiral methyl valines', i.e. samples of valine labelled stereospecifically in the methyl groups with 2H and 3H, were incorporated into cephalosporin C by a suspension of washed cells of Cephalosporium acremonium. Analysis by 3H n.m.r. of the cephalosporin C produced showed that the conversion of the 3-pro-S-methyl group of valine into the acetoxymethyl side-chain was a highly stereospecific process. By contrast, conversion of the 3-pro-R-methyl group into the endocyclic methylene group of the dihydrothiazine ring was shown to proceed by a non-stereospecific process. 相似文献
6.
7.
S. Bornemann D. H. G. Crout H. Dalton D. W. Hutchinson 《Biocatalysis and Biotransformation》1992,5(4):297-303
The hydrolysis of a prochiral diacetate by porcine pancreatic lipase is catalysed by the purified enzyme, not by an enzyme present in the crude enzyme but absent from the purified enzyme, as previously reported. 相似文献
8.
David H. G. Crout Alicia M. Dachs Susan E. Glover David W. Hutchinson 《Biocatalysis and Biotransformation》1990,4(2):177-183
The selective enzymatic hydrolysis of 3',5'-di-O-acetylthyidine (1) was studied. The lipases from porcine pancreas and Aspergillus niger, and pig liver esterase, all catalysed selective hydrolysis of the 5'O-acetyl group, but the lipase from Candida cylindracea catalysed selective hydrolysis of the 3'-O-acetyl group. Highest selectivity, leading to essentially pure 3'-O-acetylthymidine, was achieved using porcine pancreatic lipase in dilute solution at pH 7.5. Provision of an artificial interface in the form of polystyrene beads led to a significant increase in the rate of hydrolysis, accompanied by a marked fall in selectivity. Other changes in the hydrolysis conditions, such as raising the concentration of substrate or adding cosolvent, also led to a fall in selectivity. 相似文献
9.
Quantification of root water uptake in soil using X‐ray computed tomography and image‐based modelling 下载免费PDF全文
Keith R. Daly Neil M.J. Crout Stefan Mairhofer Tony P. Pridmore Sacha J. Mooney Tiina Roose 《Plant, cell & environment》2018,41(1):121-133
Spatially averaged models of root–soil interactions are often used to calculate plant water uptake. Using a combination of X‐ray computed tomography (CT) and image‐based modelling, we tested the accuracy of this spatial averaging by directly calculating plant water uptake for young wheat plants in two soil types. The root system was imaged using X‐ray CT at 2, 4, 6, 8 and 12 d after transplanting. The roots were segmented using semi‐automated root tracking for speed and reproducibility. The segmented geometries were converted to a mesh suitable for the numerical solution of Richards' equation. Richards' equation was parameterized using existing pore scale studies of soil hydraulic properties in the rhizosphere of wheat plants. Image‐based modelling allows the spatial distribution of water around the root to be visualized and the fluxes into the root to be calculated. By comparing the results obtained through image‐based modelling to spatially averaged models, the impact of root architecture and geometry in water uptake was quantified. We observed that the spatially averaged models performed well in comparison to the image‐based models with <2% difference in uptake. However, the spatial averaging loses important information regarding the spatial distribution of water near the root system. 相似文献
10.
N. A. Beresford R. W. Mayes H. S. Hansen N. M. J. Crout K. Hove B. J. Howard 《Radiation and environmental biophysics》1998,37(2):129-131
The hypothesis is tested that there is a generic relationship between the calcium intake and the transfer of radiostrontium
to milk which can be used for all dairy ruminants. In addition to the daily calcium intake, the relationship also requires
values for the strontium to calcium observed ratio, which describes the discrimination in transfer of the two elements to
milk (a value of 0.11 is used), and the calcium concentration in milk. The relationship had previously only been validated
for dairy cattle as there were insufficient data for other ruminant species. Here, we present recently available data for
dairy goats, and also a limited amount of data for sheep derived from the literature. From the comparison between these data
and predicted values, we conclude that it is possible to derive a generic model of the transfer of radiostrontium to the milk
of dairy ruminants.
Received: 12 December 1997 / Accepted in revised form: 23 March 1998 相似文献