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Daniel T Stein

Publications and source records attributed to Daniel T Stein.

10 recordsLinked to original sources
PubMed2006

High dead volume pumps: delay time reduction by short-term low-ratio split flow in LC/MS applications.

The aim of our work was to develop a low-cost, simple and reliable solution to reduce LC/MS analysis time by compensating for limitations inherent to high dead volume standard HPLC pumps. In our approach, we utilized a temporary (sub-1 min) low ratio flow split (approximately 1:10) at 5 mL/min pump flow before the column. During this short period, 90% of the entire pump flow is delivered to waste and used for fast pump/system equilibration. Although full-time flow splitting is widely used in capillary/nano applications (usually with high split ratios in the hundreds or thousands), to our knowledge, this is the first time that short-term low-ratio flow splitting has been used in conventional LC/MS applications.

Centrifugation, Density Gradient↗
PubMed2006

Sensitive quantitative analysis of C-peptide in human plasma by 2-dimensional liquid chromatography-mass spectrometry isotope-dilution assay.

BACKGROUND: Isotope-dilution assays (IDAs) are well established for quantification of metabolites or small drug molecules in biological fluids. Because of their increased specificity, IDAs are an alternative to immunoassays for measuring C-peptide. METHODS: We evaluated a 2-dimensional liquid chromatography-mass spectrometry (2D LC/MS) IDA method. Sample preparation was by off-line solid-phase extraction, and C-peptide separation was performed on an Agilent 1100 2D LC system with a purification method based on high-pressure switching between 2 high-resolution reversed-phase columns. Because of the low fragmentation efficiency of C-peptide, multiple-reaction monitoring analysis was omitted and selective-ion monitoring mode was chosen for quantification. Native and isotope-labeled ([M+18] and [M+30]) C-peptides were monitored in the +3 state at m/z 1007.7, 1013.7, and 1017.7. RESULTS: The assay was linear (r(2) = 0.9995), with a detection limit of 300 amole (1 pg) on column. Inter- and intraday CVs for C-peptide were < or =2%. Comparison with an established polyclonal-based RIA showed high correlation (r = 0.964). Plasma concentrations of total C-peptide measured by RIA were consistently higher than by IDA LC/MS, consistent with the higher specificity of IDAs compared with immunoassays. CONCLUSIONS: The 2D LC/MS IDA approach eliminates matrix effects, enhancing assay performance and reliability, and has a detection limit 100-fold lower than any previously reported LC/MS method. Isotope-labeled C-peptide(s) can be clearly differentiated from endogenous C-peptide by the difference in m/z ratio, so that both peptides can be quantified simultaneously. The method is highly precise, robust, and applicable to pharmacokinetic detection of plasma peptides.

C-Peptide↗
PubMed2006

Two-dimensional reverse phase-reverse phase chromatography: A simple and robust platform for sensitive quantitative analysis of peptides by LC/MS. Hardware design.

We have revised current two-dimensional RP-RP approaches and developed a new robust 2-D RP-RP platform. This platform was implemented on an Agilent 1100 2-D liquid chromatography system and is based on high pressure switching between two high-resolution RP columns. An independent binary gradient was implemented for each dimension. The powerful combination of dual analytical columns with independent gradient elution achieves high analyte purity, effectively eliminates matrix effects, and maximizes MS sensitivity in Q1 SIM comparable to the sensitivity enhancements of MS/MS-based methods. Implementation of dual simultaneous gradient profiles (overlapped gradients) reduces 2-D method run-time to the scale of 1-D method run-times. This robust and sensitive approach is particularly suitable for hydrophobic peptides and small proteins and can be used as a routine standard technique for enhanced on-line peptide purification coupled with mass spectrometric detection.

C-Peptide↗
PubMed2006

Health disparities between island and mainland Puerto Ricans.

OBJECTIVE: To detect health disparities among three populations--Puerto Ricans living in Puerto Rico as well as Puerto Ricans and non-Hispanic whites living on the United States (U.S.) mainland. METHODS: Data from two similarly designed surveys conducted in 1999-2000 were analyzed. The Behavioral Risk Factor Surveillance System (BRFSS) provided data on Puerto Ricans living on the island and on non-Hispanic whites in the U.S. Another survey of Puerto Ricans living in New York City provided data on mainland Puerto Ricans. The age- and sex-standardized weighted prevalences of various health parameters (e.g., obesity, diabetes, smoking, and physical illness) and indicators of access to health care (e.g., frequencies of routine checkups and diabetes care) were compared between populations by means of standardized rate ratios (SRR). RESULTS: Puerto Ricans living on the mainland and those living on the island had a similar prevalence of obesity (21% to 22%). Compared with islanders, mainland Puerto Ricans had a higher prevalence of diabetes (SRR = 1.4; 95% confidence interval [95% CI] = 1.01 to 2.0); those with diabetes also showed higher prevalences of smoking (SRR = 4.2; 95% CI = 2.3 to 7.7) and physical illness (SRR = 1.5; 95% CI = 1.1 to 2.0) than Puerto Ricans living on the island. While mainland Puerto Ricans were similar to non-Hispanic whites in terms of their utilization of primary prevention and diabetes care, island Puerto Ricans trailed behind significantly. CONCLUSIONS: Puerto Ricans living on the U.S. mainland and those living in Puerto Rico both need to target lowering their prevalence of obesity and diabetes. For island Puerto Ricans, improved education about the significance of primary prevention and diabetes care is needed. For mainland Puerto Ricans, the accessibility of the primary health care system renders it a potentially effective venue for interventions, particularly for smoking cessation. More studies are warranted to identify factors associated with the poor health status observed in mainland Puerto Ricans.

Adolescent↗
PubMed2004

A randomized trial comparing low-fat and low-carbohydrate diets matched for energy and protein.

Several recent studies have found greater weight loss at 6 months among participants on a very-low-carbohydrate (VLC) weight-loss diet compared with a low-fat (LF) weight-loss diet. Because most of these studies were not matched for calories, it is not clear whether these results are caused by decreased energy intake or increased energy expenditure. It is hypothesized that several energy-consuming metabolic pathways are up-regulated during a VLC diet, leading to increased energy expenditure. The focus of this study was to investigate whether, when protein and energy are held constant, there is a significant difference in fat and weight loss when fat and carbohydrate are dramatically varied in the diet. The preliminary results presented in this paper are for the first four of six postmenopausal overweight or obese participants who followed, in random order, both a VLC and an LF diet for 6 weeks. Other outcome measures were serum lipids, glucose, and insulin, as well as dietary compliance and side effects. Our results showed no significant weight loss, lipid, serum insulin, or glucose differences between the two diets. Lipids were dramatically reduced on both diets, with a trend for greater triglyceride reduction on the VLC diet. Glucose levels were also reduced on both diets, with a trend for insulin reduction on the VLC diet. Compliance was excellent with both diets, and side effects were mild, although participants reported more food cravings and bad breath on the VLC diet and more burping and flatulence on the LF diet.

Adipose Tissue↗
PubMed2002

Bulk magnetic susceptibility effects on the assessment of intra- and extramyocellular lipids in vivo.

Localized proton spectroscopy provides a novel method for noninvasive measurement of lipid content in skeletal muscle. It has been suggested that the chemical shift difference between lipid signals from distinct compartments in skeletal muscle might be caused by bulk magnetic susceptibility (BMS) differences from lipids stored in intra- (IMCL) and extramyocellular (EMCL) compartments. Direct evidence is provided to confirm the theoretical prediction that compartment symmetry is responsible for discrimination between resonances of IMCL and EMCL. Phantoms imitating lipids in skeletal muscle were constructed using soybean oil to represent EMCL, and IntralipidTM, an intravenous fat emulsion of fine droplets, to represent IMCL. It was found that the chemical shift of IntralipidTM is independent of the BMS effects, while the resonance of soybean oil shifts in a predictable manner determined by the geometry of the compartment.

Lipids↗
PubMed2002

MRI of muscular fat.

An MRI technique with high selectivity and sensitivity to the signal components in the chemical shift range of methylene and methyl protons of fatty acids has been developed for noninvasive assessment of muscular fat in vivo. A spoiled gradient-echo sequence with spatial-spectral excitation by six equidistant pulses with 2 degrees -(-9 degrees )-17 degrees -(-17 degrees )-9 degrees -(-2 degrees ) and a multi-echo train (TE = 16, 36, 56, 76, 96, and 116 ms) allowed a series of images to be recorded with a receiver bandwidth of 78 Hz per pixel. SIs from phantoms with lipid contents between 0.1% and 100% were compared to those from pure water. Thirty healthy volunteers underwent fat-selective imaging of their lower leg, and parallel localized proton spectroscopy of the tibialis anterior and the soleus muscle by a single-voxel stimulated echo acquisition mode (STEAM) technique (TR = 2 s, TE = 10 ms, TM = 15 ms). Results show a high correlation (r = 0.91) between fat imaging and the spectroscopic approach in the soleus muscle, considering the percentage total fat content of musculature. The correlation coefficient was clearly lower (r = 0.55) in the tibialis anterior muscle due to signal contaminations from adjacent subcutaneous fat in the images, inhomogeneous fat distribution, and generally lower lipid content in this muscle. Applications of the new imaging technique showed marked intra- and interindividual variability in the spatial distribution of lipids in the musculature of the lower leg. No significant correlation of the muscular fat with the thickness of the subcutaneous fat layer was found. In addition, the body mass index does not appear to determine muscular fat content, except in very obese cases.

Adult↗
PubMed2002

The effectiveness of the use of electrocautery with chondroplasty in treating chondromalacic lesions: A randomized prospective study.

PURPOSE: To determine the clinical effect of electrocautery with arthroscopic chondroplasty on chondromalacic tissue. TYPE OF STUDY: A randomized, prospective study. METHODS: In a prospective study, 146 patients were randomly placed in either a control group (chondroplasty alone) or a treatment group (chondroplasty and electrocautery). Intraoperative findings in both treatment and control groups were classified by both the extent of chondromalacia and the number of compartments in which chondromalacia was diagnosed. In our study, we used 6 defined compartments: medial and lateral tibial plateau, medial and lateral femoral condyles, patella, and femoral trochlea. Patients were followed up at 1 year and evaluated using the Lysholm scale. Postoperative Lysholm scores were matched against compartment involvement and compared between groups. RESULTS: There was no significant difference in the patients with grade 2 chondromalacia in either control or cautery group (P >.5). Groups with grade 3 chondromalacia showed a significant difference favoring the control group (P <.05). A significant difference was found in comparing 2-compartment chondromalacia between the 2 treatment cohorts, again, favoring the control group (P =.01). The remaining groups of multiple compartment chondromalacia showed no significant difference, indicating that the postoperative result was independent of the use of electrocautery. CONCLUSIONS: This study shows that electrocautery as an adjunct to chondroplasty offers little benefit in the treatment of chondromalacic lesions and may, in fact, limit successful outcome.

Arthroscopy↗
PubMed2002

Acute elevation of NEFA causes hyperinsulinemia without effect on insulin secretion rate in healthy human subjects.

Increased circulating levels of nonesterified free fatty acids (NEFA) have been observed in such hyperinsulinemic states as obesity, impaired glucose tolerance, diabetes, and dyslipidemia where they have been causally linked to the development of insulin resistance and hyperinsulinemia. The concentration of NEFA in plasma is believed to have direct modifying effects on insulin secretion and clearance. It remains controversial whether acute increases in NEFA potentiate insulin secretion in human subjects. We studied the effect of an acute elevation of NEFA during lipid-heparin infusion compared to a glycerol-only control on glucose-stimulated insulin secretion and clearance during a 120-min hyperglycemic (10 mM) clamp in 7 healthy normoglucose-tolerant volunteers. The metabolic clearance rate of C-peptide (MCR(CP)) was measured in each subject during the study by simultaneous infusion of C-peptide. Insulin secretion rate (ISR) was calculated from deconvolution of C-peptide data after correction for the rate of C-peptide infusion. Clearance rate of insulin (MCR(INS)) was calculated based upon endogenous ISR. Plasma glucose (mg/dL): basal (90-115 min) 90.2 +/- 2.8 vs. 90.2 +/- 2.3; clamp (150-240 min) 180.5 +/- 2.8 vs. 180.9 +/- 1.3. Plasma insulin (pmol/L): prebasal (fasting) 29.6 +/- 10.0 vs. 29.8 +/- 10.6; basal (90-115 min) 30.1 +/- 9.2 vs. 34.5 +/- 12.1; second phase clamp (210-240 min) 127.6 +/- 18.2 vs. 182.5 +/- 17.3*. Plasma NEFA (mM): prebasal 0.47 +/- 0.08 vs. 0.52 +/- 0.09; basal 0.35 +/- 0.05 vs. 0.98 +/- 0.02*; clamp (122-240 min) 0.06 +/- 0.02 vs. 0.77 +/- 0.06*. ISR (pmol/min): prebasal 72.7 +/- 7.5 vs. 72.0 +/- 7.9; second phase clamp (210-240 min) 268.5 +/- 27.2 vs. 200.2 +/- 23.7. MCR(INS) (mL/min): prebasal 3393 +/- 488 vs. 3370 +/- 511; clamp 2284 +/- 505 vs. 1214 +/- 153* (*p < 0.05 glycerol vs. intralipid/heparin). This study demonstrates that acute NEFA elevation causes hyperinsulinemia due to a significant decrease in systemic insulin clearance without increasing rates of insulin secretion.

Adult↗