Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Limitations”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2Linked to original sources
PubMed1978

Oxygen-limited continuous culture and respiratory energy conservation in Escherichia coli.

Escherichia coli B was cultured continuously in succinate-minimal medium under conditions of oxygen limitation in the phauxostat. With decreasing oxygenation and consequent decreasing growth rates, the complement of terminal cytochrome oxidases changed as follows: high growth rates, cytochrome o; intermediate growth rates, cytochromes o and d; lowest growth rates, cytochromes o, d, and a1. Respiratory kinetics exhibited by nongrowing cell suspensions obtained from continuous cultures indicated that terminal oxidase activity was exhibited by cytochrome o (Km for O2 = 0.2 micron; Vmax = 1.1 to 1.5 mumol of O2 per nmol of cytochrome o per min) and cytochrome d (Km for O2 = 0.024 micron; Vmax = 0.7 mumol of O2 per nmol of cytochrome d per min). During oxygen-limited growth, the molar growth yield referred to respiration, and corrected for maintenance respiration [Yo(max)], was 12.6 g (dry weight) per g-atom of oxygen, not significantly different from the succinate-limited value of 12.0 g (dry weight) per g-atom of oxygen. The rate of maintenance respiration of the oxygen-limited culture was only 3.4 mg-atoms of O per g (dry weight) per h, some threefold less than that of the succinate-limited culture. Respiration-driven proton extrusion did not vary with the growth rate or with the complement of terminal oxidases (H+/O = 3.7; standard deviation, 0.07). We conclude that the content of terminal oxidases is without effect on the efficiency of respiratory energy conservation.

Cytochromes↗
PubMedDate not supplied

[Effect of yeast growth limitation on the respiratory chain, the economic coefficient and the critical oxygen concentration for respiration].

The effect of the yeast growth limitation by oxygen on the economical coefficient (EC), the operation of the cyanide resistant electron transport pathway (CrETP), and the critical for respiration oxygen concentration concentration ([O2]cr) was studied. The operation of CrETP was found to differ among various yeasts growing on glucose: it could function during both the exponential phase and limitation of growth (Torulopsis candida), or only in the conditions of growth limitation (Candida tropicalis, C. mycoderma, C. lipolytica), sometimes for a very long period (Endomyces geotrichum); in certain cases (C. utilis), it cannot be detected at all. If the main respiratory chain is inhibited by cyanide (i. e. if only CrETP operates), the value of [O2]cr sharply increases; such an increase can be also found in the absence of cyanide but in the conditions of active operation of CrETP. Apparently, the value of [O2]cr is higher for cyanide resistant oxydase of the studied organisms than for cytochrome oxydase. A decrease in EC observed upon the limitation of yeast growth by oxygen (Lozinov et al., 1974) correlates with the appearance or intensification of CrETP. Therefore, the decrease of EC can be attributed to the operation of non-phosphorylating CrETP which occurs in all the studied yeasts (with an exception of C. utilis) when their growth is limited by oxygen.

Candida↗
PubMed1979

The effectiveness of time-limited psychotherapy in a university counseling center.

Investigated the effects of eight-session, time-limited therapy on clients' self-concepts. Fourty-two students who sought psychotherapy or personal counseling at the university counseling center were assigned randomly to either time-limited, time-unlimited, or control groups after screening with the MMPI to eliminate the most severely disturbed clients. The Bills Index of Adjustment and Values was administered pre- and post-therapy. Results of a two-way repeated measures analysis indicated significant improvement in both time-limited and unlimited groups, but not in the controls. The two treatment groups did not differ in improvement. A 2 1/2 year follow-up of 18 of the former clients suggested that treatment gains were maintained and were equivalent for those who received time-limited and open-ended therapy.

Counseling↗
PubMed1977

Influence of specific growth limitation and dilution rate on the phosphorylation efficiency and cytochrome content of mitochondria of Candida utilis NCYC 321.

With Candida utilis cells that had been removed directly from a 61 chemostat culture, in steady state, well-coupled mitochondria generally could be isolated. This requires a modified snail-gut enzyme procedure that allowed the total processing time to be decreased to 3 h, or less. Examination of these mitochondria in an oxygraph showed the presence of 3 sites of energy conservation when the cells were grown at various dilution rates between 0.1 and 0.45 h-1 in environments that were, successively, glucose-, ammonia-, magnesium-, phosphate- and sulphate-limited. Potassium-limited cells also apparently possessed 3 sites of oxidative phosphorylation when growing at dilution rates greater than 0.2 h-1, but only 2 sites when growing at lower dilution rates. Analysis of cytochrome spectra obtained with these intact mitochondria revealed large quantitative (but not qualitative) differences, depending on the environmental conditions under which the yeast had been cultured. In particular, comparison of the ratio of cytochrome b to cytochrome a showed a pattern of change with dilution rate in mitochondria from potassium-limited cells that was distinctly different from those evident in mitochondria from cells that had been limited in their growth by the availability of other nutrients.

Ammonia↗
PubMed1979

Satellite DNA properties of the germ line limited DNA and the organization of the somatic genomes in the nematodes Ascaris suum and Parascaris equorum.

During the early cleavage divisions in some Ascarids, parts of the chromosomes are eliminated from the somatic blastomeres ("chromatin diminution", Boveri, 1887) while the chromosomes in the germ line cells maintain their integrity. To characterize the germ line and soma genome, DNA was isolated from gametes and embryonic somatic cells of two Ascarid species, Parascaris equorum var. univalens and Ascaris suum. It was shown that the germ line limited DNAs of these species have the same density and almost identical reassociation kinetics: in CsCl the predominant component of the germ line limited DNA of P. equorum and A. suum has the buoyant density of 1.697 g/cm3, while soma DNA of both species bands at 1.700 g/cm3. In P. equorum there is a small additional germ line limited satellite DNA component with the density of 1.690 g/cm3, identical to that of mitochondrial DNA of both organisms. Comparison of the reassociation kinetics of germ line and soma DNA demonstrates for both species that the eliminated DNA sequences are highly repetitive. In contrast to these similarities between the germ line limited DNAs of P. equorum and A. suum the analysis of their base composition revealed differences (40% guanine plus cytosine in P. equorum and 36% in A. suum). The only very fast reassociating DNA sequences which we could isolate from soma DNA was demonstrated to be foldback DNA. The reassociation kinetics of total A. suum soma DNA was investigated by hydroxylapatite chromatography. Least squares analysis of the data revealed about 10% of intermediate repetitive DNA sequences. Their interspersion between single copy DNA sequences was analyzed by comparing the reassociation kinetics of DNA fragments 0.35 and 7.2 kilobases long. Thus the DNA sequence arrangement of Ascaris does not follow the short period interspersion pattern observed in most organism.

Animals↗
PubMed2025

Genomic prediction of agronomic traits in perennial ryegrass (Lolium perenne L.) and genotype x environment interactions at the limit of the species distribution.

KEY MESSAGE: Perennial ryegrass shows extensive genotype x environment interactions at the limit of its ecological niche. Accounting for GxE may improve prediction even when environmental and genetic samples are highly diverse. BACKGROUND: In breeding the aim is to identify and accumulate beneficial variants. However, detection of these variants may be challenging in the presence of extensive genotype x environment interactions (GxE). METHODS: The study assesses the performance of 264 diploid perennial ryegrass accessions in a multi-environment field trial. We investigate the extent of GxE, for yield (total dry matter) and persistence traits under environmental conditions experienced in Nordic and Baltic regions at the limit of the species distribution. Two different approaches to modelling GxE were tested and validated under three different breeding scenarios. RESULTS: Our analysis documented the presence of significant GxE for all traits. Validation showed improvements in prediction accuracy when accounting for GxE: up to 4% for yield when predicting in unobserved environments, and up to 22% and 9% for spring cover and winter kill, respectively, when predicting unobserved germplasm. Genome-wide-association-studies (GWAS) were utilized to detect genetic variants with marginal effects (environment-independent effect) and conditional effects (environment-dependent effects). Results showed the presence of large-effect genetic variants with marginal effects, in addition to few Quantitative Trait Loci (QTL) whose effects were adaptive under specific environmental conditions while neutral or deleterious under different environmental conditions. CONCLUSION: This study demonstrates the usefulness and limitations of genomic prediction models for predicting GxE in highly diverse samples and describes the extent of GxE at the limit of species distribution for perennial ryegrass. Our study points towards adaptive variation which may enhance persistence of perennial ryegrass populations in Nordic and Baltic growing conditions.

Lolium↗
PubMed1976

Vitamin B-6-catalyzed beta-elimination of serine and O-phosphoserine. Qualitative and quantitative aspects of catalytic influences at the rate-limiting step, a comparison with the rate of enzymatic beta-elimination.

The overall reaction rates for the beta-elimination of serine and O-phosphoserine, catalyzed by various vitamin B-6 analogs (pyridoxal 5'-phosphate, 5'-deoxypyridoxal and N-methylpyridoxal 5'-phosphate) in the presence or absence of Cu2+ ions, are determined. The comparison of the pH-dependence of the molar activities of the three vitamin B-6 aldehydes in beta-elimination of serine enables the characterization of the different active Schiff base species and the single catalytic events. The Schiff base which has a positive charge on the pyridine ring nitrogen and a fully ionized phosphate group shows the highest molar activity. The phosphate group acts as an intramolecular general base catalyst, most probably at the alpha-carbon proton of the amino acid. Furthermore general acid catalysis by buffer species occurs at the beta-hydroxy group serine. These facts together provide a kinetically unambiguous description of the mechanism of the reaction: the removal of the proton at the alpha-carbon atom of serine is the rate-limiting step and is followed by the more rapid elimination of the b-hydroxy group of serine. The forward rate constant of the rate-limiting step is calculated for each of the reactions mentioned. The rate constants are compared with respect to the effectiveness of the individual catalytic components in the vitamin B-6-dependent beta-elimination. For optimal conditions the reaction of O-phosphoserine is faster by a factor of 10(4) in the velocity of the beta-elimination than the corresponding acid-catalyzed beta-elimination of serine. For the eliminations at the alpha- and beta-carbon atoms of O-phosphoserine in vitamin B-6-catalysed reactions a common transition state is discussed. From a comparison of the fastest vitamin beta-6-dependent model reaction with the rate of an enzymatic beta-elimination it is suggested that for those beta-elininating enzymes where the rate-limiting step is the same as in the model, the catalytic components mentioned could suffice to explain the velocity of the rate-limiting step.

Binding Sites↗
PubMed1979

Change from homo- to heterolactic fermentation by Streptococcus lactis resulting from glucose limitation in anaerobic chemostat cultures.

Lactic streptococci, classically regarded as homolactic fermenters of glucose and lactose, became heterolactic when grown with limiting carbohydrate concentrations in a chemostat. At high dilution rates (D) with excess glucose present, about 95% of the fermented sugar was converted to l-lactate. However, as D was lowered and glucose became limiting, five of the six strains tested changed to a heterolactic fermentation such that at D = 0.1 h(-1) as little as 1% of the glucose was converted to l-lactate. The products formed after this phenotypic change in fermentation pattern were formate, acetate, and ethanol. The level of lactate dehydrogenase, which is dependent upon ketohexose diphosphate for activity, decreased as fermentation became heterolactic with Streptococcus lactis ML(3). Transfer of heterolactic cells from the chemostat to buffer containing glucose resulted in the nongrowing cells converting nearly 80% of the glucose to l-lactate, indicating that fine control of enzyme activity is an important factor in the fermentation change. These nongrowing cells metabolizing glucose had elevated (ca. twofold) intracellular fructose 1,6-diphosphate concentrations ([FDP](in)) compared with those in the glucose-limited heterolactic cells in the chemostat. [FDP](in) was monitored during the change in fermentation pattern observed in the chemostat when glucose became limiting. Cells converting 95 and 1% of the glucose to l-lactate contained 25 and 10 mM [FDP](in), respectively. It is suggested that factors involved in the change to heterolactic fermentation include both [FDP](in) and the level of lactate dehydrogenase.

Acetates↗
PubMed1979

Response of Rhodopseudomonas capsulata to illumination and growth rate in a light-limited continuous culture.

Rhodopseudomonas capsulata was grown under anaerobic, photosynthetic conditions in a continuous culture device. Under light limitation, at a constant dilution rate, it was shown that cell composition, including photopigment (bacteriochlorophyll and carotenoids) and ribonucleic acid content, was not affected by incident light intensity; however, steady state culture density varied directly and linearly with light intensity. On the other hand, photopigment and ribonucleic acid levels were affected by growth rate regardless of light intensity. Additional experiments indicated a high apparent Ks for growth of R. capsulata with respect to light. These results were interpreted to mean that near the maximum growth rate (D = 0.45 h-1) some internal metabolic process became the limiting factor for growth, rather than the imposed energy limitation. A mathematical expression for the relation between steady-state culture density and dilution rate was derived and was found to adequately describe the data. A strong correlation was found between continuous cultures limited either by light or by a chemical energy source.

Bacteriochlorophylls↗
PubMed1977

Radiorespirometry of Candida utilis in phased culture under nitrogen-, carbon-, and phosphorus-limited growth.

The release of 14CO2 from specifically labelled glucose ([G-1-14C],[G-2-14C],[G-3,4-14C], and [G-6-14C]) by phased cells of C. utilis was examined at intervals during 6-h cycles under conditions of N-,P-, and C-limited growth. On the basis that the release of 14CO2 from [G-1-C14] could serve as a measure of hexose monophosphate pathway (HMP) activity, of 14CO2 from [G-3,4-14C] as a measure of Embden-Meyerhof-Parnas (EMP) activity, and 14CO2 from [G-6-14C] as indicative of tricarboxylic acid (TCA) cycle activity, it was concluded that a relatively uniform operation of EMP occurred throughout the cycle in N-, P-, and C-limited cells, and of HMP and TCA in C-limited cells, but considerable variations took place in HMP and TCA cycle activities in N- and P-limited cells. The patterns of 14CO2 released from [G-2-14C] were more closely related to those observed from [G-1-14C] and [G-3,4-14C] than from [G-6-14C]. Changes in the rate of 14CO2 evolution during the cycle were sometimes abrupt and likely coincided with 'critical points' of metabolic activity in the cycle.

Candida↗
PubMed1979

Wind speed limits to work under hot environments for clothed men.

Four heat-acclimated clothed young adult men exercised (treadmill) at metabolic rate of 191 W.m-2 (27% VO2 max), under five air temperatures (Ta) between 36 and 53 degrees C and three wind velocities (v), 1, 2, and 4 m.s-1, for 2 h. The 2nd h of each experiment involved progressive increases in the ambient vapor pressure (Pa) to force an upward inflection of the rectal temperature (Tre). The Tre point of inflection identified the critical Pa (Pcrit) for each Ta. The average mean skin temperature (Tsk = 36 degrees C for all Pcrit. Straight-line isotherms for Tsk = 36 degrees C, which agreed with the negative regressions of the Pcrit on Ta, represented the limits of exposure for Ta less than or equal to 44 degrees C. The slope characteristics of the isotherms corresponded with skin wettedness (w) of 0.94, 0.71, and 0.58, respectively, for v of 1, 2, and 4 m.s-1. For Ta greater than 44 degrees C the limit line corresponded with steeper negative regressions indicating a lower w than for Ta less than or equal to 44 degrees C. Despite the increase in the ambient evaporative capacity due to the higher v the limit lines describing exposure limits were not significantly different either at Ta less than or equal to 44 degrees C for v of 2 and 4 m.s-1 or at Ta greater than 44 degrees C for all three v.

Acclimatization↗
PubMedDate not supplied

[Limiting behavior of a model of the impulse activity of a neuron for large frequency of input flow and small contributions of a single synapse (non-diffusion approximation)].

The limit behavior of probability models of neuron's firing, viz. models with exponential decay and constant threshold is considered. These models differ from each other by F(v), distribution of jumps of potential after arrival of input impulse. If average number of input impulses arriving during the decaying time of one impulse in very large and threshold depolarisation, measured by unit of average jump of potential, is large too, than limit density of interspike interval distribution does not depend on form of F(v); and depends on other neuron's parameters mean and variance of F(v). Moreover, the form of this density depends on only one parameter--normalised threshold. Under this condition interspike interval distribution for diffusion model converges to the same limit. Lapase transform for limit distribution is found and results of its numeric inverts are given.

Action Potentials↗
PubMed1979

Influence of a between-run component of variation, choice of control limits, and shape of error distribution on the performance characteristics of rules for internal quality control.

A computer-stimulation study has been performed to determine how the performance characteristics of quality-control rules are affected by the presence of a between-run component of variation, the choice of control limits (calculated from within-run vs. total standard deviations), and the shape of the error distribution. When a between-run standard deviation (Sb) exists and control limits are calculated from the total standard deviation (St, which includes Sb as well as the within-run standard deviation, Sw), there is generally a loss in ability to detect analytical disturbances or errors. With control limits calculated from Sw, there is generally an increase in the level of false rejections. The presence of non-gaussian error distribution appears to have considerably less effect. It can be recommended that random error be controlled by use of a chi-square or range-control rule, with control limits calculated from Sw. Optimal control of systematic errors is difficult when Sb exists. An effort should be made to reduce Sb, and this will lead to increased ability to detect analytical errors. When Sb is tolerated or accepted as part of the baseline state of operation for the analytical method, then further increases in the number of control observations will be necessary to achieve a given probability for error detection.

Chemistry, Clinical↗
PubMed2026

Prevalence and risk factors of red blood cell alloimmunization among sickle cell disease patients in resource-limited countries: A systematic review and meta-analysis.

BACKGROUND: Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by hemoglobin S production, in which homozygous individuals (HbSS) develop a broad range of acute and chronic complications. While disease-modifying and curative therapies are increasingly available in high-income settings, red blood cell (RBC) transfusion remains the mainstay of treatment in resource-limited countries and is associated with high rates of alloimmunization. This systematic review and meta-analysis aimed to estimate the prevalence of alloimmunization and identify associated risk factors among patients with SCD living in resource-limited settings. METHODS: Africa Journals Online (AJOL), Embase, PubMed, Scopus, and Web of Science were searched for original studies published from inception to December 15, 2025. Only studies conducted in low- and lower-middle-income countries (LMICs) were included. Eligible studies evaluated the prevalence of alloimmunization in patients with SCD receiving RBC transfusions. A random-effects meta-analysis of proportions was performed to pool quantitative data, while qualitative findings were systematically summarized in tabular form. Statistical heterogeneity was assessed using the I² statistic and further explored using Baujat plots, leave-one-out analyses, and meta-regression. RESULTS: Our analysis included 27 studies conducted in Africa (n = 23) and Asia (n = 4), predominantly from lower-middle-income countries (n = 19) and mainly employing a cross-sectional design (n = 20), comprising 3128 previously transfused patients with SCD. The pooled prevalence of RBC alloimmunization was 8.76% (95% CI: 6.71-11.37%; I² = 75%). Higher alloimmunization rates were observed in West and North Africa, particularly in Côte d'Ivoire, Egypt, and Nigeria, whereas lower rates were reported in Asia and East Africa. The most frequently identified antibodies belonged to the Rh blood group system (n = 153), followed by the Kell system (n = 65). CONCLUSION: In resource-limited settings, RBC alloimmunization is a frequent and clinically significant complication in patients with SCD, contributing to increased morbidity and potential mortality. Targeted and economically viable antigen matching may reduce alloimmunization rates and improve transfusion safety in LMICs.

Humans↗
PubMed2026

A scalable HPC framework for bioinformatics in resource-limited settings: design principles, implementation, and sustainability from the UVRI experience.

MOTIVATION: Building and sustaining High-Performance Computing (HPC) infrastructure for bioinformatics research in resource-limited settings presents significant technical, financial and operational challenges. Institutions in low-and middle-income regions often face constraints such as limited technical expertise, unstable infrastructure and restricted funding which can hinder the deployment of large-scale computational platforms necessary for modern genomics and bioinformatics analyses. RESULTS: We present a scalable and modular HPC framework developed at the Uganda Virus Research Institute (UVRI) to support large-scale genomics and other omics data analyses in resource-limited settings. The framework integrates open-source HPC management tools, infrastructure automation, and reproducible configuration management to enable reliable deployment and maintenance. Optimized storage and networking configurations combined with a phased capacity-building strategy support high-throughput genomic workflows while strengthening local technical expertise. From our implementation experience, we derive ten practical design and operational rules that provide a transferable methodology for establishing and sustaining in-house HPC infrastructure. These rules emphasize strategic investment in human capacity, structured planning, leveraging collaborations, adoption of open-source technologies and service management practices to improve operational resilience and long-term sustainability. AVAILABILITY: The design principles, automation strategies and implementation guidelines described in this work are applicable to institutions seeking to establish sustainable HPC resources for bioinformatics research in resource-constrained environments.

Computational Biology↗
PubMed1977

Elastic tissue in the limiting membrane of the human seminiferous tubule.

The structure and distribution of elastic tissue were studied in the limiting membrane of the seminiferous tubule from normal human testis. The elastic and elastic-related fibers (oxytalan and elaunin) were recognized by their tinctorial and ultrastructural characteristics. The connective structures of the limiting membrane, including the fibrotubules and the amorphous material of the elastic system, were studied after tannic acid-glutaraldehyde fixation. Fibrotubules (oxytalan fibers) were observed in almost all intercellular spaces of the limiting membrane, closely related to the contractile cells; the elaunin fibers (patches of amorphous material surrounded by bundles of fibrotubules) were evident in the outermost layers. The function of this system of elastic tissue and myoid cells is discussed, considering the permeability membrane and the role of the myoid cells in the elastogenesis and contractility of the seminiferous tubule.

Elastic Tissue↗
PubMed1978

Testing with puromycin and amino acyl tRNAs that limit the rate of peptide chain extension.

With puromycin one can recognize when the synthesis of a given protein is dependent on amino acyl tRNA that is present in rate limiting amount. We demonstrate this use of puromycin by its interaction with another inhibitor, L-o-methylthreonine. L-o-methylthreonine lowers the Ile-tRNA concentration in the cell, thereby inhibiting synthesis of proteins containing isoleucine. In certain rabbits, the alpha hemoglobin chain has three isoleucyl residues and the beta chain none. L-o-methylthreonine thus inhibits alpha globin synthesis in intact reticulocytes from these rabbits. When puromycin and L-o-methylthreonine are used together, the two inhibitors synergize in inhibiting alpha globin synthesis. Hence, puromycin is a more effective inhibitor when the Ile-tRNA concentration is lowered. Cycloheximide and sodium fluoride have different modes of action from puromycin. Neither synergizes with L-o-methylthreonine; instead, the interaction is less than additive. We have found that beta chain synthesis in rabbit reticulocytes is more sensitive than alpha to inhibition by puromycin. This difference could reflect either differences in amino acid sequence or tRNA dependent limitations of beta chain elongation. The switch from fetal to adult hemoglobin in humans does not involve changes in limiting amino acyl tRNA because, for cord blood from infants of different developmental ages, the puromycin sensitivity of incorporation into gamma and beta chains remains constant.

Animals↗
PubMed1978

Biomass yields of Chlorella from iron (Yx/Fe) in iron-limited batch cultures.

The maximum biomass in iron-limited photosynthetic batch cultures of chlorella increased as the logarithm of the iron concentration. The growth yield from iron (UxFe) showed a marked inverse relation to the specific growth rate. The maximum biomass yield, g dry biomass/g iron consumed, was 7.5 X 10(3) with specific growth rate 0.108h-1; the minimum was 0.79 X 10(3) with specific growth rate 0.145 h-1. The maximum specific growth rate in the exponential phase of Fe limited cultures varied as the initial Fe concentration. Fe-limited growth made the cells adhere to a glass surface.

Chlorella↗