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F Svec

Publications and source records attributed to F Svec.

At least 73 records · Page 4Linked to original sources
PubMed1996

New designs of macroporous polymers and supports: from separation to biocatalysis.

Reactive polymers play many roles, from supports for solid-phase synthesis or catalysis to media for separations. Although macroporous polymer beads that provide high reactive capacities and excellent solvent tolerance are well established, approaches to monosized beads with optimized pore structures or multiple chemistries segregated within pores of different sizes have expanded their realm of application. Polymer monoliths containing intricate pore networks can be obtained in any desired shape by a simple molding process and provide unique advantages such as fast kinetics, high reactivity, and high throughput. Applications ranging from immobilized enzyme reactors to fast media for the separation of synthetic or biopolymers are presented.

Catalysis↗
PubMed1996

Molded monolithic rod of macroporous poly(styrene-co-divinylbenzene) as a separation medium for HPLC of synthetic polymers: on-column precipitation--redissolution chromatography as an alternative to size exclusion chromatography of styrene oligomers and polymers.

A process for the separation of styrene oligomers and polymers by size and composition using a novel separation medium has been demonstrated. The process involves precipitation of the macromolecules on the molded macroporous rod columns, followed by progressive elution utilizing a simple gradient of the mobile phase. Molded macroporous rod columns are ideally suited for this technique because convection through the large pores of the rod enhances the mass transport of large analyte molecules and accelerates the separation process. Styrene oligomers and polymers are separated in a 50-mm x 8-mm-i.d. column using a solvent gradient composed of a poor solvent such as water, methanol, or acetonitrile and increasing amounts of a good solvent, tetrahydrofuran. Excellent separations are obtained, demonstrating that precipitation-redissolution can be a suitable alternate to size exclusion chromatography (SEC) of some polymers. Compared to SEC, the gradient elution separation can be achieved at higher flow rates in a much shorter time. Precipitation-redissolution with gradient elution can also be used for the separation of copolymers, for which the process is controlled not only by molecular weight but also by the composition of the copolymers.

Chromatography, Gel↗
PubMed1996

Differential stimulation of cortisol and dehydroepiandrosterone levels by food in obese and normal subjects: relation to body fat distribution.

BACKGROUND: It has been previously shown that food intake elevates circulating ACTH and cortisol levels, but no report has been published regarding the changes in circulating dehydroepiandrosterone (DHEA). DHEA was originally described as a weak androgen, but more recently it has been associated with a wide range of metabolic functions. In addition, previous studies have described a hyper-responsive hypothalamo-pituitary-adrenal axis in obese subjects in response to various stimuli, but the specific response to food has not been studied. SUBJECTS AND DESIGN: We studied the effect of food on the hypothalamo-pituitary-adrenal axis in 20 subjects of normal body mass index (BMI range 18-25) and also in a group of 12 obese subjects (BMI range 34-61). Levels of glucose, insulin, ACTH, cortisol and dehydroepiandrosterone were measured every 20 minutes. RESULTS: A small rise in DHEA accompanies the rise in circulating ACTH and cortisol in response to food in both lean and obese subjects, but DHEA rose independently of cortisol and ACTH on the fasting day. In the obese subjects, food induced a significantly greater change in serum cortisol (peak cortisol rise (mean +/- SEM); normal-weight group, 169 +/- 14%; obese group, 294 +/- 23%) and in the cortisol/DHEA ratio (area under the curve; normal-weight group, 202 +/- 15%; obese group, 292 +/- 29%) than in the normal-weight subjects. This difference was particularly notable in those with central-type obesity (waist/hip ratio > 0.80). A group of the normal, jean female subjects showed no cortisol rise after food intake. CONCLUSION: Our results suggest that DHEA may vary independently of circulating cortisol, and that the cortisol response to food is enhanced in obese subjects, particularly in those with central obesity. We speculate that there may be a caused connection between the cortisol response to food in normal subjects, and the subsequent distribution of fat if such subjects overeat sufficiently to become obese.

Adipose Tissue↗
PubMed1996

Pheochromocytoma with elevated metanephrines as the only biochemical finding.

Pheochromocytoma is usually diagnosed by finding elevated levels of metanephrines, catecholamines, or vanillylmandelic acid in 24-hour urine collections. At least two of the three urine analytes are usually elevated. We found no report of a documented pheochromocytoma in which only one of the three analytes was elevated. Herein we report a patient with severe hypertension and a right adrenal mass, who was found on repeated testing to have only elevated urinary metanephrines. The 24-hour urine collections for vanillylmandelic acid, total catecholamines, fractionated catecholamines, and urine dopamine were all normal. The patient underwent surgery for the right adrenal mass, and the pathology report confirmed the diagnosis of pheochromocytoma.

Adrenal Gland Neoplasms↗
PubMed1995

A paradoxical elevation of brain cyclo(His-Pro) levels in hyperphagic obese Zucker rats.

Several studies suggest a role for endogenous cyclo(His-Pro) or CHP in appetite regulation. In the present study, we have examined the regional brain distribution of CHP in hyperphagic obese Zucker rats and their lean littermates. The data show a significant elevation in the levels of CHP in many brain regions, including hypothalamus of the obese rat. Within the hypothalamus, the lateral hypothalamic (LH) nucleus of obese rats had significantly higher levels of CHP when compared to that of the lean littermates. Administration of dehydroepiandrosterone, a steroid hormone known to decrease food intake and body weight gain, to obese rats led to decrease in the levels of CHP in the LH. These data further suggest a role for the endogenous CHP in attenuating food intake.

Animals↗
PubMed1995

Modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) continuous rod columns for preparative-scale ion-exchange chromatography of proteins.

A continuous rod of porous poly(glycidyl methacrylate-co-ethylene dimethacrylate) has been prepared by a free radical polymerization within the confines of a 300 x 8 mm I.D. chromatographic column. The epoxide groups of the rod have been modified by a reaction with diethylamine that affords ionizable functionalities required for the ion-exchange chromatographic mode. The properties of this rod column have been characterized and the column has been used successfully for the chromatographic separation of proteins. The column exhibits a dynamic capacity that exceeds 300 mg at a flow velocity of 200 cm/min. An excellent selectivity allows the separation of up to 300 mg of a protein mixture in a single run.

Animals↗
PubMed1995

Hydrophilization of porous polystyrene-based continuous rod column.

Reactive porous rods of poly[(chloromethyl)styrene-co-divinylbenzene] have been prepared for the first time by copolymerization of the monomers in the presence of a porogenic diluent. A two-step modification process involving reaction with ethylenediamine followed by reaction with gamma-gluconolactone leads to a porous medium that has highly hydrophilic surface functionalities. Preliminary chromatographic evaluation with both small molecules such as alkylbenzenes in reversed-phase mode and large proteins in hydrophobic adsorption and ion-exchange modes demonstrates that the surface of the modified rods possesses a hydrophilicity that is comparable to that of the best hydrophilic HPLC packings.

Chromatography, High Pressure Liquid↗
PubMed1995

Uniform-size hydrophobic polymer-based separation media selectively modified with a hydrophilic external polymeric layer.

A simple procedure for the preparation of macroporous hydrophobic styrene-divinylbenzene polymeric separation media with a hydrophilic outer surface has been developed. A hydrophilic monomer and water-soluble polymerization initiator are added to the reaction mixture during the final polymerization step of the preparation of size-monodisperse particles. Because the hydrophobic styrene-divinylbenzene framework of the beads is already formed, and the hydrophilic monomer does not penetrate the pores of the beads that are filled with a hydrophobic porogen, the hydrophilic layer is formed only at the surface of the beads. The hydrophilic monomers used included glycerol monomethacrylate and glycerol dimethacrylate and toluene was used as the porogen for the poly(styrene-co-divinylbenzene) beads. Comparative experiments involving beads with or without a hydrophilic medium showed that the separation selectivity of the media towards hydrophobic solutes remains unchanged. However, the modified medium with a hydrophilic layer could be used to analyse mixtures that also contained large peptide molecules as these do not adsorb at its surface as is the case with the unmodified hydrophobic beads.

Chromatography, Liquid↗
PubMed1995

Dehydroepiandrosterone and macronutrient selection by obese Zucker rats (fa/fa).

The effect of dehydroepiandrosterone (DHEA) on the macronutrient preference and total energy intake of lean and obese female Zucker rats was studied. Introduction of DHEA led to a rapid decrease in the consumption of fat, protein and total calories by the obese rat. DHEA treatment of the lean rat caused a change neither in its total energy consumption nor in its fat consumption, but did cause a distinct expression of preference for carbohydrate over protein. Both lean and obese animals gained weight rapidly during the steroid-free weeks and lost weight while consuming the DHEA-supplemented diet. The difference in weight changes seen with the obese rats between the DHEA-free and DHEA-supplemented periods could be accounted for by differences in caloric intake. Lean rats, however, did not change their total energy intake during DHEA-treatment periods; therefore, DHEA caused weight loss in the lean rat probably by altering energy metabolism. It is concluded that in the obese, young, female Zucker rat, DHEA alters macronutrient preference as well as caloric intake. In the lean rat, DHEA has a more subtle effect on relative macronutrient preference and not on total energy consumption.

Animals↗
PubMed1995

The effect of DHEA given chronically to Zucker rats.

Dehydroepiandrosterone (DHEA) has been reported to exert antiglucocorticoid activity. When administered to obese, hypercorticosteronemic Zucker rats, it causes a diminution of food intake and a reduction in their rate of weight gain. This experiment was conducted to evaluate whether this biologic effect could be ascribed to chronic adrenal insufficiency. Obese and lean Zucker rats were treated with DHEA as a food supplement for 28 days. Upon sacrifice, organ weights and serum chemistries were measured, along with neurotransmitter levels in regions of the hypothalamus. Results showed that although the obese animals gained weight more slowly, had lower insulin levels, and ate less, their serum glucose, corticosterone, and ACTH levels were not different from control. Hypothalamic neurotransmitters in the obese rat were unaffected by chronic DHEA treatment. We concluded that, although DHEA clearly affects Zucker weight gain, it does not induce chronic adrenal insufficiency.

Adrenal Glands↗
PubMed1995

The effect of discontinuing dehydroepiandrosterone supplementation on Zucker rat food intake and hypothalamic neurotransmitters.

OBJECTIVE: Dehydroepiandrosterone (DHEA) decreases body weight and food intake of the obese Zucker rat, a model of youth-onset obesity associated with hyperphagia. The effects of discontinuing DHEA treatment on these parameters, however, has not been investigated. This question was studied in rats that had been maintained on DHEA-supplemented (0.0%, 0.06%, 0.15%, 0.3% or 0.6%) diets for 7 days. METHOD: The results were correlated with regional levels of hypothalamic neurotransmitters in rats treated with 0.6% DHEA for 7 days in a separate experiment. Neurotransmitter changes were evaluated after Day 0 (7 days of treatment), and Day +1 and Day+2 post-DHEA. RESULTS: Upon removing dietary DHEA, rats immediately (+1 day) consumed significantly more food than while on the DHEA-supplemented diet. Indeed, they consumed even more food than the group that had always been on the DHEA-free diet. This intake above control lasted for as long as +9 days post-DHEA treatment. After 7 days of DHEA treatment, lateral hypothalamic (LH) serotonin (5-HT) and dopamine (Dpm) were elevated significantly (P < 0.05) immediate changes in 5-HT and Dpm returned to baseline by day 2 of post-DHEA treatment. No significant changes occurred in either the ventromedial hypothalamus (VMH) or the paraventricular nucleus (PVN). CONCLUSIONS: These observations suggest that there is a possible relationship between increases of LH 5-HT and Dpm with 0.6% DHEA treatment. Both are inhibitory to food intake and DHEA at the 0.6% dose causes hypophagia after 7 days of treatment (i.e. 0 days). Subsequent decreases of these monoamines occurred during the post-DHEA period at both +1 and +2 days. Return of these inhibitory monoamines to baseline could be responsible for reversal of the hypophagia, however, they do not rule out the production of a separate stimulator of food intake.

Androstenedione↗
PubMed1995

Pituitary tumor and low gonadotropins in a patient with Turner's syndrome.

Despite years of prolonged hypersecretion of gonadotropins, patients with Turner's syndrome rarely develop nonfunctional pituitary tumors. We present here the case of an 18-year-old woman who had both Turner's syndrome and a pituitary tumor. The patient was diagnosed at age 16 with Turner's syndrome, after presenting with primary amenorrhea. She was subsequently found to have inappropriately low levels of luteinizing hormone and follicle-stimulating hormone. Workup confirmed the unique combination of Turner's syndrome, a pituitary tumor, low gonadotropins and high androgen levels.

Adenoma↗
PubMed1995

Central effects of dehydroepiandrosterone in Zucker rats.

OBJECTIVES: To investigate whether dehydroepiandrosterone (DHEA), an adrenal/gonadal androgen, can act centrally to reduce energy intake in a model of genetic obesity, the Zucker fatty rat. To investigate a possible mechanism of action. DESIGN: Two experiments were performed in lean and obese female Zucker rats. In the first experiment, 24 h following administration of i.p. DHEA (200 mg/kg), three hypothalamic regions [lateral hypothalamus (LH), ventromedial nucleus (VMH), and paraventricular nucleus (PVN)] were analyzed for monoamine neurotransmitter concentrations. In the second experiment, DHEA (50 micrograms) was administered by i.c.v. injection. Energy intake for the following day was measured. MEASUREMENTS: In the first experiment, concentrations of norepinephrine (NE), epinephrine (EPI), dopamine (DA), serotonin (5HT), the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured. Ratios of 5HT/5HIAA were calculated. In the second experiment, kilojoules consumed per 24 h were calculated. RESULTS: All LH monoamines, and PVN DA, displayed lower concentrations in obese than lean control rats. DHEA treatment reversed these reductions in obese rats without affecting lean rats. DHEA increased VMH EPI in obese rats only. DHEA increased PVN NE in both lean and obese rats. I.C.V. DHEA decreased energy intake in obese but not lean rats. CONCLUSION: The i.c.v. results suggest that DHEA exerts a phenotype specific, centrally mediated inhibitory effect on food intake. In addition, in doses previously shown to reduce energy intake in obese but not lean rats, i.p. DHEA reversed reduced concentrations of many monoamines, particularly in the LH, in obese animals only. These latter changes provide indirect evidence to suggest that these central neurotransmitters may play an important role in the antiobesity effect of DHEA in the Zucker fatty rat.

Animals↗
PubMed1994

Two-dimensional high-performance liquid chromatography using monodisperse polymer beads containing segregated chemistries prepared by pore size specific functionalization. Single-column combinations of size exclusion or ion exchange with reversed-phase chromatography.

Separation media for the complete separation of complex samples that require a combination of size exclusion or ion-exchange with reversed-phase chromatographic modes in a single column have been prepared from size monodisperse 10 microns poly(glycidyl methacrylate-co-ethylene dimethacrylate) beads using a pore size specific functionalization process. To achieve the first combination of chromatographic modes, the large pores of the beads were selectively hydrolyzed to diols using aqueous poly(styrenesulfonic acid), while highly hydrophobic octadecyl groups were introduced into the small pores by reaction of the remaining epoxide groups with octadecylamine. These beads provide excellent protein recoveries and may be used for the direct injection separation of samples containing both hydrophilic proteins and hydrophobic drugs. Beads containing diethylamino groups in the large pores and octadecyl functionalities in the small pores were also prepared by size-selective modification. A plot of log k' against ionic strength of the mobile phase for these beads shows the absence of hydrophobic interactions and documents the clean ion-exchange mechanism of protein separation. Examination of the small pores in both types of separation media confirmed that their hydrophobicity was sufficient to allow the separations of small molecules in reversed-phase mode. Column packed with these dual-chemistry beads exhibited high efficiencies and were used successfully for the separations of proteins and alkylbenzenes or drugs.

Animals↗
PubMed1994

High-performance liquid chromatography of complex mixtures using monodisperse dual-chemistry polymer beads prepared by a pore-size-specific functionalization process. A single column combination of hydrophobic interaction and reversed-phase chromatography.

A novel separation medium for HPLC combining hydrophobic interaction and reversed-phase separation modes in a single column has been prepared from monodisperse 10-microns poly-(glycidyl methacrylate-co-ethylene dimethacrylate) beads using a pore-size-specific functionalization process. In this approach, the large pores of each bead were provided with phenyl groups interspersed among hydrophilic functionalities while a much higher surface concentration of hydrophobic phenyl groups was introduced into the small pores. Due to the size-specific character of the modification process, no protein interaction with any highly hydrophobic surface was observed during chromatography. The beads were used for the separation of samples containing both proteins and small hydrocarbon or drug molecules. A plot of log k' against salt concentration in the mobile phase clearly documents the clean hydrophobic interaction mechanism of protein separation and the absence of charged groups while the linear plot of log k' against acetonitrile concentration for numerous compounds demonstrates the reversed-phase separation ability. No decrease of the efficiency of the test column (23,000 plates/m) was observed in long-term experiments during which more than 1000 injections and many changes between the modes were performed.

Chemical Phenomena↗
PubMed1994

Reversed-phase chromatography of small molecules and peptides on a continuous rod of macroporous poly(styrene-co-divinylbenzene).

A continuous "molded" rod of porous poly(styrene-co-divinylbenzene) prepared by a bulk free radical polymerization within the confines of a chromatographic column has been used successfully for the reversed-phase HPLC of alkylbenzenes and peptides. An excellent rapid separation of bradykinin and [D-Phe7]-bradykinin with a molecular mass of about 1000 was also achieved.

Amino Acid Sequence↗