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Man Ho Choi

Publications and source records attributed to Man Ho Choi.

8 recordsLinked to original sources
PubMed2005

Characterization of testosterone 11 beta-hydroxylation catalyzed by human liver microsomal cytochromes P450.

A combination of accelerator mass spectrometry (AMS) and liquid chromatography-tandem mass spectrometry has been used to clarify some new aspects of testosterone metabolism. The main pathway of testosterone oxidative metabolism by human liver microsomes is the formation of 1beta-, 2alpha-/beta-, 6beta-, 15beta-, and 16beta-hydroxytestosterones, mainly catalyzed by cytochromes P450 2C9, 2C19, and 3A4. We now report the first determination that 11beta-hydroxytestosterone (11beta-OHT) can also be formed by human liver microsomal fractions. The structures of five hydroxylated metabolites of testosterone (2beta-, 6beta-, 11beta-, 15beta-, and 16beta-OHT) and the C-17 oxidative metabolite androstenedione were determined by liquid chromatography with UV detection at 240 nm and liquid chromatography-tandem mass spectrometry. Corresponding results were obtained by high-performance liquid chromatography-AMS analysis of incubations of [4-14C]testosterone with human liver microsomes. 6beta-Hydroxylation was always the dominant metabolic pathway, but 2beta-, 15beta-, and 16beta-OHT, and androstenedione were also formed. The previously undetected hydroxytestosterone, 11beta-OHT, was found to be a minor metabolite formed by human liver microsomal enzymes. It was formed more readily by CYP3A4 than by either CYP2C9 or CYP2C19. 11beta-Hydroxylation was inhibited by ketoconazole (IC50 = 30 nM) at concentrations similar to the IC50 (36 nM) for 6beta-hydroxylation Therefore, CYP3A4 could be mainly responsible for testosterone 11beta-hydroxylation in the human liver. These findings identify human hepatic biotransformation of testosterone to 11beta-OHT as a previously unrecognized extra-adrenal metabolic pathway.

Catalysis↗
PubMed2004

Two-dimensional protein separation with advanced sample and buffer isolation using microfluidic valves.

Methods are described to achieve more efficient multidimensional protein separation in a microfluidic channel. The new methods couple isoelectric focusing (IEF) with high ionic strength electrophoretic separations by active microvalve control in a microchip. Several experiments demonstrating independent 2D separation were performed, and critical parameters for optimal chip performance were identified, including channel passivation, electroosmosis control, and IEF linearity control. This strategy can be used for integration of different heterogeneous separation techniques, such as IEF, capillary electrophoresis, and liquid chromatography. This new device can be ideal for preseparation and preconcentration of complex biomolecule samples for a streamlined biomolecule analysis using mass spectrometry.

Buffers↗
PubMed2003

Direct determination of estriol 3- and 16-glucuronides in pregnancy urine by column-switching liquid chromatography with electrospray tandem mass spectrometry.

Using column-switching liquid chromatography/tandem mass spectrometry (LC-MS/MS), we developed an improved analytical method of urinary estriol glucuronides. This new method is derived predominantly from maternal and fetal precursors in pregnancy. We used in the following procedure: first, we filtered urine samples with a membrane filter. Next, we directly injected the 50 microL aliquot of urine samples onto a pre-column. Then, after activating the column-switching valve, we backflushed the loaded samples onto the C(18) analytical column. Urine samples can be assayed within 20 min without any sample preparation steps. We monitored separated estriol glucuronides by negative electrospray ionization (ESI) and selected-reaction monitoring (SRM). The calibration range of estriol-3-glucuronide (E3-3G) and estriol-16-glucuronide (E3-16G) was 0.1-20 microg/mL and the linearity of the method was 0.9984 for E3-3G and 0.9987 for E3-16G. The limits of detection at a signal-to-noise (S/N) ratio of 3 were 10 ng/mL (E3-3G) and 5 ng/mL (E3-16G). The analytical recovery was over 85% and, in general, inter-day and intra-day variability for precision and accuracy were less than 10%. When applied to a pregnancy urine sample to biomedical monitoring of the function of the maternal/fetal unit, the proposed method allowed rapid and sensitive screening for the detection of E3-3G and E3-16G.

Adult↗
PubMed2002

Rapid and sensitive determination of sertraline in human plasma using gas chromatography-mass spectrometry.

A method for the determination of sertraline in human plasma using gas chromatography-mass spectrometry (GC-MS), with the selected ion-monitoring (SIM) mode, was described. The following was used in this study: (1) single liquid-liquid extraction at alkaline pH after deproteinization of plasma protein and (2) perfluoroacylation with HFBA, which has higher sensitivity (about 10-fold) compared with previous reported derivatization. The detection limit for the SIM of sertraline as an N-HFB derivative was 0.1 ng/ml, and its recovery was 80-85%. The linear response was obtained in the range of 0.2-10.0 ng/ml with a correlation coefficient of 0.999. The coefficient of variation (C.V.%) was less than 12.1% in the 1-30 ng/ml, and less than 18.2% at 0.2 ng/ml, and the accuracy was less than 10% at all of the concentration range. These findings indicate that this assay method has adequate precision and accuracy to determine the amount of sertraline in human plasma. After pharmacokinetics was performed with this assay method following oral administration of sertraline hydrochloride in man, moment analysis revealed that pharmacokinetic parameters for sertraline (Cmax, 10.3 ng/ml; Tmax, 8.0 h; T(1/2) 28.6 h) were similar to previously reported results. These results indicate that this simple and sensitive assay method is readily applicable to the pharmacokinetic studies of sertraline.

Adult↗
PubMed2002

Determination of non-steroidal estrogens in breast milk, plasma, urine and hair by gas chromatography/mass spectrometry.

It is suspected that all the natural estrogens occurring in the human body, as well as dietary and synthetic estrogens, diversely affect the endocrine system depending on their exposure patterns. More rapid, reliable and accurate measurements of these compounds in various biological matrices are thus becoming an important task. After solid-phase extraction using an Oasis HLB extraction cartridge, the estrogen concentrates were derivatized with a mixture of N-methyl-N-trifluorotrimethylsilylacetamide/ammonium iodide/dithioerythritol (1000:4:5, v/w/w) for analysis by gas chromatography/mass spectrometry in the selected ion-monitoring (SIM) mode. The qualitative identification of estrogens detected in SIM mode was further confirmed by tandem mass spectrometry using low-energy collision-induced dissociation (CID) mode. The method for the assay of the 20 estrogens was linear over the ranges of 1-1000 micro g/L for biological fluids and 1-200 micro g/kg for hair with high correlation coefficient (>0.99). The limits of quantitation (LOQ) ranged from 1.0-10 micro g/L (or micro g/kg) and the limit of detection ranged from 0.2-3 micro g/L (or micro g/kg). The average precision (% CV) and accuracy (% bias) of the method determined at the LOQ, low, and medium concentrations were in the ranges 2.6-9.2 and -4.1-7.7, respectively. The average extraction recovery of the estrogens from plasma and hair at the three concentration levels varied in the ranges 77-103% (1.9-14.3% CV) and 73-104% (3.1-14%), respectively. The distribution patterns of the estrogens were characteristic of each biosample. Five estrogens in the range 1.5-44.9 micro g/L were measured in breast milk, 8 estrogens in the range 3.5-322 micro g/L in plasma, 12 estrogens at 1.2-442 micro g/L in urine, and biochanin-A at 13.2-39.1 micro g/kg in hair. Because of its high sensitivity, good precision and specificity, the present method was found suitable for the trace analysis of dietary and synthetic estrogens in complex biosamples such as breast milk, plasma, urine and hair.

Adult↗
PubMed2002

Measurement of corticoids in the patients with clinical features indicative of mineralocorticoid excess.

BACKGROUND: A method for the measurement of five important serum and urinary corticoids on the syndrome of mineralcorticoid excess is reported. The methodology was combined gas chromatography-mass spectrometry (GC-MS) with selected ion-monitoring mode. METHODS: After extraction with a solid-phase cartridge using an Oasis HLB copolymer, the residues were derivatized with a mixture of N-methyl-N-trimethylsilyltrifluoroacetamide/ammonium iodide/dithioerythritol (1000:4:5, v/w/w), and analyzed. RESULTS: The linearity as the regression coefficients were >0.979 over a range of 1-500 ng/ml, and limit of detection ranged from 1 to 3 ng/ml while their analytical recoveries varied in the range of 75.7-94.9%. The overall precision (% CV) of the method were 3.2-7.2% and 3.6-6.3% for serum and urine, respectively. The accuracy expresses as % bias ranged from -4.1 to 6.4%. This assay was used on two patients with hypokalemic hypertension, and may be useful in ruling out mineralcorticoid excess (AME) type 1 or 2. CONCLUSIONS: The present GC-MS technique may be useful to differentiate between the syndrome of AME and other hypertensive diseases with clinical features suggestive of mineralcorticoid excess because of the assay's reliablity and precision.

Adrenal Cortex Hormones↗
PubMedDate not supplied

Comparative plasma proteome analysis of lymphoma-bearing SJL mice.

In SJL mice, growth of RcsX lymphoma cells induces an inflammatory response by stimulating V(beta)16+ T cells. During inflammation, various serum protein levels can increase (e.g., acute phase reactants) or decrease (e.g., albumin), and most of these altered proteins are thus potential biomarkers. Although blood plasma is a valuable and promising sample for biomarker discovery for diseases or for novel drug targets, its proteome is complex. To address this, we have focused on a comprehensive comparison of the plasma proteomes from normal and RcsX-tumor-bearing SJL mice using the 1D-Gel-LC-MS/MS method after removing albumin and immunoglobulins. This analysis resulted in the identification of a total of 1079 nonredundant mouse plasma proteins; more than 480 in normal and 790 in RcsX-tumor-bearing SJL mouse plasma. Of these, only 191 proteins were found in common. The molecular weights ranged from 2 to 876 kDa, covering the pI values between 4.22 and 12.09, and included proteins with predicted transmembrane domains. By comparing the plasma proteomic profile of normal and RcsX-tumor-bearing SJL mice, we found significant changes in the levels of many proteins in RcsX-tumor-bearing mouse plasma. Most of the up-regulated proteins were identified as acute-phase proteins (APPs). Also, several unique proteins i.e., haptoglobin, proteosome subunits, fetuin-B, 14-3-3 zeta, MAGE-B4 antigen, etc, were found only in the tumor-bearing mouse plasma; either secreted, shed by membrane vesicles, or externalized due to cell death. These results affirm the effectiveness of this approach for protein identification from small samples, and for comparative proteomics in potential animal models of human disorders.

14-3-3 Proteins↗