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Biomedical subjects

Stephen R Dueker

Publications and source records attributed to Stephen R Dueker.

11 recordsLinked to original sources
PubMed2006

Human vitamin B12 absorption measurement by accelerator mass spectrometry using specifically labeled (14)C-cobalamin.

There is a need for an improved test of human ability to assimilate dietary vitamin B(12). Assaying and understanding absorption and uptake of B(12) is important because defects can lead to hematological and neurological complications. Accelerator mass spectrometry is uniquely suited for assessing absorption and kinetics of carbon-14 ((14)C)-labeled substances after oral ingestion because it is more sensitive than decay counting and can measure levels of (14)C in microliter volumes of biological samples with negligible exposure of subjects to radioactivity. The test we describe employs amounts of B(12) in the range of normal dietary intake. The B(12) used was quantitatively labeled with (14)C at one particular atom of the dimethylbenzimidazole (DMB) moiety by exploiting idiosyncrasies of Salmonella metabolism. To grow aerobically on ethanolamine, Salmonella enterica must be provided with either preformed B(12) or two of its precursors, cobinamide and DMB. When provided with (14)C-DMB specifically labeled in the C2 position, cells produced (14)C-B(12) of high specific activity (2.1 GBq/mmol, 58 mCi/mmol) (1 Ci = 37 GBq) and no detectable dilution of label from endogenous DMB synthesis. In a human kinetic study, a physiological dose (1.5 microg, 2.2 kBq/59 nCi) of purified (14)C-B(12) was administered and showed plasma appearance and clearance curves consistent with the predicted behavior of the pure vitamin. This method opens new avenues for study of B(12) assimilation.

Carbon Radioisotopes↗
PubMed2004

Quantitation of in vivo human folate metabolism.

BACKGROUND: A quantitative understanding of human folate metabolism is needed. OBJECTIVE: The objective was to quantify and interpret human folate metabolism as it might occur in vivo. DESIGN: Adults (n = 13) received 0.5 nmol [(14)C]pteroylmonoglutamate (100 nCi radioactivity) plus 79.5 nmol pteroylmonoglutamate in water orally. (14)C was measured in plasma, erythrocytes, urine, and feces for >/=40 d. Kinetic modeling was used to analyze and interpret the data. RESULTS: According to the data, the population was healthy and had a mean dietary folate intake of 1046 nmol/d, and the apparent dose absorption of (14)C was 79%. The model predictions showed that only 0.25% of plasma folate was destined for marrow, mean bile folate flux was 5351 nmol/d, and the digestibility of the mix (1046 + 5351 nmol/d) was 92%. About 33% of visceral pteroylmonoglutamate was converted to the polyglutamate form, most of the body folate was visceral (>99%), most of the visceral folate was pteroylpolyglutamate (>98%), total body folate was 225 micromol, and pteroylpolyglutamate synthesis, recycling, and catabolism were 1985, 1429, and 556 nmol/d, respectively. Mean residence times were 0.525 d as visceral pteroylmonoglutamate, 119 d as visceral pteroylpolyglutamate, 0.0086 d as plasma folate, and 0.1 d as gastrointestinal folate. CONCLUSIONS: Across subjects, folate absorption, bile folate flux, and body folate stores were larger than prior estimates. Marrow folate uptake and pteroylpolyglutamate synthesis, recycling, and catabolism are saturable processes. Visceral pteroylpolyglutamate was an immediate precursor of plasma p-aminobenzoylglutamate. The model is a working hypothesis with derived features that are explicitly model-dependent. It successfully quantitated folate metabolism, encouraging further rigorous testing.

Adult↗
PubMed2004

Phytochemical research using accelerator mass spectrometry.

Vegetables and fruits provide an array of microchemicals in the form of vitamins and secondary metabolites (phytochemicals) that may lower the risk of chronic disease. Tracing these phytochemicals at physiologic concentrations has been hindered by a lack of quantitative sensitivity for chemically equivalent tracers that could be used safely in healthy people. Accelerator mass spectrometry is a relatively new technique that provides the necessary sensitivity (in attomoles) and measurement precision (<3%) towards 14C-labeled phytochemicals for detailed kinetic studies in humans at dietary levels.

Food Analysis↗
PubMed2003

Absorption and retinol equivalence of beta-carotene in humans is influenced by dietary vitamin A intake.

The effect of vitamin A supplements on metabolic behavior of an oral tracer dose of [14C]beta-carotene was investigated in a longitudinal test-retest design in two adults. For the test, each subject ingested 1 nmol of [14C]beta-carotene (100 nCi) in an emulsified olive oil-banana drink. Total urine and stool were collected for up to 30 days; concentration-time patterns of [14C]beta-carotene, [14C]retinyl esters, and [14C]retinol were determined for 46 days. On Day 53, the subjects were placed on a daily vitamin A supplement (10000 IU/day), and a second dose of [14C]beta-carotene (retest) was given on Day 74. All 14C determinations were made using accelerator mass spectrometry. In both subjects, the vitamin A supplementation was associated with three main effects: 1). increased apparent absorption: test versus retest values rose from 57% to 74% (Subject 1) and from 52% to 75% (Subject 2); 2). an approximately 10-fold reduction in urinary excretion; and 3). a lower ratio of labeled retinyl ester/beta-carotene concentrations in the absorptive phase. The molar vitamin A value of the dose for the test was 0.62 mol (Subject 1) and 0.54 mol (Subject 2) vitamin A to 1 mol beta-carotene. Respective values for the retest were 0.85 and 0.74. These results show that while less cleavage of beta-carotene occurred due to vitamin A supplementation, higher absorption resulted in larger molar vitamin A values.

Absorption↗
PubMed2002

Method for the simultaneous determination of retinol and beta-carotene concentrations in human tissues and plasma.

To understand differential tissue distribution of retinoids and carotenoids, as it might influence biological processes in humans, we developed and demonstrated a method for measuring them in selected human tissues. The method includes internal standards and a secondary reference standard to eliminate the need for external standard calibration and to minimize sample-handling errors. Tissues were digested (saponified) in ethanolic KOH. Retinol and beta-carotene were extracted with organic solvent containing internal standards. Analytes were separated using isocratic liquid chromatography and quantified at 325 nm for retinol and 450 nm for beta-carotene. Plasma was analyzed in a similar way but without saponification. Retinal-O-ethyloxime and beta-apo-12'-carotenal-O-t-butyloxime served as internal standards. Plasma, breast, and fat from breast surgery patients and colon, liver, muscle, and fat from colon surgery patients were analyzed. Within-day relative standard deviations (RSDs) for plasma were <0.04 for beta-carotene and <0.03 for retinol, between-day RSDs were <0.05 for beta-carotene and <0.04 for retinol. Saponification ensured complete extraction of retinol and beta-carotene and removal of triglycerides that "foul" chromatographic columns. It seems retinol and beta-carotene concentrations in tissues and blood of cancer patients are the same or higher than those in corresponding tissues of patients without these cancers.

Adult↗
PubMed2002

A parallel processing solid phase extraction protocol for the determination of whole blood folate.

We describe an improved whole blood folate analysis method that facilitates increased throughput compared to our previous method (Dueker et al. (2000) Anal. Biochem. 283, 266). Improvements include three items: first, a buffered solvent exchange to remove interfering amino acids, especially phenylalanine whose esters may interfere with the analysis because their retention times on the gas chromatography are close to those of the para-aminobenzoic acid (pABA) isotopomers; second, substituting an NH2 solid phase extraction step for an HPLC step permits the batch parallel processing of samples; third, replacing trifluoroacetyl derivatives of ethyl-esterified pABA isotopomers with heptafluorobutyl derivatives, which are better resolved on the GC column. The method measures pABA, a stable degradation product of folate. This simplifies sample handling and purification. Relative standard deviations are typically 5% or less and a single operator can process samples in batches of 40. Results from our GCMS method correlate (R = 0.98) with the Lactobacillus casei assay for whole blood folate. The modifications will facilitate the development of high throughput methods for whole blood folate. Our method holds promise for epidemiological and clinical studies, where accurate whole blood folate concentrations are needed. Because it is internally standardized, interlaboratory variation should be minimal.

4-Aminobenzoic Acid↗
PubMed2002

Dual isotope test for assessing beta-carotene cleavage to vitamin A in humans.

BACKGROUND: The ability of beta-carotene to deliver bioactive retinoids to tissues is highly variable. A clearer understanding of the environmental and genetic factors that modulate the vitamin A potential of beta-carotene is needed. AIM OF STUDY: Assess the vitamin A value of orally administered beta-carotene relative to a co-administered reference dose of preformed vitamin A. METHODS: Equimolar doses (30 micromol) of hexadeuterated D6 beta-carotene and D6 retinyl acetate were orally co-administered in an emulsified formulation to a male subject. The plasma concentration time courses of D6 retinol (derived from D6 retinyl acetate) and bioderived D3 retinol (from D(6) beta-carotene) were determined for 554 h postdosing using gas chromatography/mass spectrometry. Intact D6 beta-carotene plasma concentrations were determined by high-pressure liquid chromatography. The ratio of the two forms of vitamin A, D6 retinol/D3 retinol, at any single time point is postulated to reflect the quantity of vitamin A derived from beta-carotene relative to preformed vitamin A. Additionally, a minute amount of 14C beta-carotene (50 nCi; 0.27 microg) was included in the oral dose and cumulative 24-h stool and urine samples were collected for two weeks to follow absorption and excretion of the b-carotene. The 14C nuclide was detected using accelerator mass spectrometry (AMS). Results During the absorption/distribution phase (3-11 h) the D6/D3 ratio of the two retinols was not stable and ranged between a value of 3 and 16. Between 11 and 98 h postdosing the ratio was relatively stable with a mean value of 8.5 (95 % CI: 7.5, 8.7). These data suggest that in this subject and under these conditions, 8.5 moles of beta-carotene would provide a vitamin A quantity equivalent to 1 mole of preformed vitamin A. On a mass basis, 15.9 microg of beta-carotene was equivalent to 1 microg of retinol. The total administered beta-carotene was found to be 55 % absorbed by AMS analysis of cumulative stool. CONCLUSION: The co-administration of D6 beta-carotene and D6 retinyl acetate provides a technique for assessing individual ability to process beta-carotene to vitamin A. The results indicate that a single time point taken between 11-98 h after dose administration may provide a reliable value for the relative ratio of the two forms of vitamin A. However, results from more subjects are needed to assess the general utility of this method.

Adult↗
PubMed2002

Plasma beta-carotene and retinol concentrations of children increase after a 30-d supplementation with the fruit Momordica cochinchinensis (gac).

BACKGROUND: In rural Vietnam, vitamin A deficiency is a concern. Among the indigenous fruit and vegetables, Momordica cochinchinensis (gac) fruit has been identified as having the highest beta-carotene concentration. Locally, it is mixed with rice in a preparation called xoi gac. OBJECTIVE: The purpose of this study was to assess this beta-carotene- rich rice preparation as a source of provitamin A for children in rural Vietnam. DESIGN: Preschoolers (n = 185) participated in a 30-d controlled supplementation trial. Children with low hemoglobin concentrations were assigned to 1 of 3 groups: a fruit group, who received xoi gac that contained 3.5 mg beta-carotene per serving; a powder group, who received rice mixed with 5.0 mg synthetic beta-carotene powder; and a control group, who received rice without fortification. RESULTS: The mean increase in plasma beta-carotene concentrations in the fruit and powder groups was significantly greater than that in the control group (P < 0.0001). After supplementation, the mean plasma retinol concentration in the fruit group was significantly higher than that in the control (P = 0.006) and powder (P = 0.0053) groups. Among the children with initial hemoglobin concentrations <or=110 g/L, the mean increase in hemoglobin concentrations in the fruit group was marginally higher than that in the control group (P = 0.017) but was not significantly different from that in the powder group. CONCLUSIONS: beta-Carotene from xoi gac is a good source of provitamin A carotenoids. Severely anemic children might particularly benefit from routine xoi gac consumption

Anthropometry↗
PubMed2002

Variability in conversion of beta-carotene to vitamin A in men as measured by using a double-tracer study design.

BACKGROUND: The vitamin A activity of beta-carotene is variable and surprisingly low in women. The reasons for this are not well understood. The vitamin A activity of beta-carotene in men is still uncertain. Contributions of dietary factors compared with individual traits are largely unknown. OBJECTIVE: Our objective was to measure the intrinsic variability in the vitamin A activity of beta-carotene among healthy, well-fed men living in a controlled environment. DESIGN: We used a double-tracer test-retest design. We dosed 11 healthy men orally with 30 micromol hexadeuterated (D6) retinyl acetate (all-trans-19,19,19,20,20,20-[2H6]retinyl acetate) and then with 37 micromol D6 beta-carotene (19,19,19,19',19',19'-[2H6]beta-carotene) 1 wk later. Doses were taken with breakfasts containing 16 g fat. We measured D6 retinol, D6 beta-carotene, and trideuterated (D3) retinol (derived from D6 beta-carotene) concentrations in plasma. Areas under the plasma concentration x time since dosing curves (AUCs) were determined for D6 retinol, D6 beta-carotene, and D3 retinol. RESULTS: All men had detectable D6 retinol concentrations in plasma. The mean (+/-SE) absorption of D6 beta-carotene in all subjects was 2.235 +/- 0.925%, and the mean conversion ratio was 0.0296 +/- 0.0108 mol retinol to 1 mol beta-carotene. Only 6 of 11 men had sufficient plasma concentrations of D6 beta-carotene and D3 retinol that we could measure. The mean absorption of D6 beta-carotene in these 6 subjects was 4.097 +/- 1.208%, and the mean conversion ratio was 0.0540 +/- 0.0128 mol retinol to 1 mol beta-carotene. CONCLUSION: The vitamin A activity of beta-carotene, even when measured under controlled conditions, can be surprisingly low and variable.

Absorption↗
PubMed2002

Semiparametric modeling of labeled-cell kinetics, with application to isotope labeling of erythrocytes.

We propose a stochastic model for the kinetics of cells that have been tagged with a chemical label. The proposed model consists of two components: a parametrically specified distribution for the time to incorporation of the label into the cells and a nonparametric survival function reflecting the survival time of the label-cell combination. The target quantity of this modeling approach is the fraction of labeled cells among all cells, viewed as a function of time. Longitudinal measurements of this labeled-cell fraction are available from a recent experiment with folate-labeled red blood cells. The proposed semiparametric model is fitted to these data and some of the implications are explored. The proposed method also includes bootstrap-based inference.

Carbon Radioisotopes↗