The assay had a lower limit of determination of 50 pmol

The assay had a lower limit of determination of 50 pmol. Km Vand value for these substrates in human liver. the livers of male Wistar rats (200C300 g) which were sacrificed by cervical dislocation. Enzyme assays for CYP probes a) Phenacetin O-deethylationInitial linearity studies indicated that this reaction was linear up to 1 1.5 mg protein for human and 2.0 mg protein for rat liver microsomes and an incubation time of 30 min in both species. A 500 l reaction mixture typically containing 0.5 mg microsomal protein (human or rat) was incubated with phenacetin in the presence of MgCl2 (10 mm) and NADPH (2.5 mm) in phosphate buffer (0.067 m; pH 7.4). Metacetamol was added as internal standard and the mixture was extracted with DCM (10 ml; 20 min) to remove unreacted phenacetin followed by ethylacetate (10 ml; 20 min). Samples were reconstituted in mobile phase (200 l) prior to h.p.l.c. analysis. Paracetamol and metacetamol were separated using an isocratic mobile phase (flow rate 1 ml min?1) consisting of AcN: sodium phosphate buffer (10:90, v/v; 0.1 m; pH 4.3) and a Spherex 5 C18 column (25 cm x SPHINX31 4.6 mm; Phenomenex, Macclesfield, UK) with u.v. detection at 245 nm. Formation of paracetamol was quantified by interpolating peak height ratios of paracetamol and SPHINX31 metacetamol from a standard curve of known paracetamol concentrations. Inter- and intra- assay coefficients of variation were 8.7% and 6.5% (determined at 100 pmol paracetamol) respectively. The lower limit of determination was 25 pmol. SPHINX31 b) Tolbutamide 4-hydroxylationInitial linearity studies indicated that this reaction was linear up to 4 mg and 2 mg microsomal protein for human and rat liver microsomes respectively and an incubation time of 16 min for human liver microsomes and 10 min for rat liver microsomal preparations. A 500 l reaction mixture containing 0.5 mg microsomal protein (human or rat) was incubated with tolbutamide in the presence of MgCl2 (10 mm) and NADPH (1 mm) in phosphate buffer (0.067 m; pH 7.4) according to the method of Back [21]. Termination, extraction SPHINX31 and h.p.l.c. analysis of samples were as previously described. The inter- and intra- assay coefficients of variation (determined at 0.6 nmol hydroxytolbutamide) were 3% and 1.3% respectively, with a lower limit of determination of 20 pmol. c) Chlorzoxazone 6-hydroxylationInitial linearity studies revealed that this reaction was linear upto 2 mg microsomal protein in both species and an incubation time of 40 min. A 500 l incubation volume containing 0.2 mg microsomal protein (human or rat) was incubated with chlorzoxazone (CLZ) in the presence of MgCl2 (10 mm) and NADPH (1 mm) in phosphate buffer (0.067 m; pH 7.4). The reaction was terminated by the addition of zoxazolamine as internal standard and the mixture was extracted with DCM (5 ml; 10 min). The organic phase was evaporated to dryness and reconstituted into mobile phase (200 l) prior to h.p.l.c. analysis. Formation of 6-hydroxychlorzoxazone (6-OHCLZ) was measured by h.p.l.c. with u.v. detection at 295 nm and quantified by interpolating peak height ratios of 6-OHCLZ and zoxazolamine from a standard curve of known 6-OHCLZ concentrations. A 5 C18 Spherex column (25 cm 4.6 mm; Phenomenex, Macclesfield, UK) was employed to separate CLZ, 6OHCLZ and internal standard using a gradient mobile phase system. Initial chromatographic conditions were AcN:ammonium acetate buffer (28:22, v/v; 0.05; pH 3.3), followed by a linear increase of AcN to 33% between 10 and 15 min remaining so until 17 min then returning to the original run conditions at 20 min. This was followed by a 5 min re-equilibration period. The inter- and intra- assay coefficients of variation (determined at 3 and 10 nmol 6-OHCLZ) were 5.3% and 8.5% respectively. The lower limit of determination was 100 pmol. Rabbit Polyclonal to CSGALNACT2 d) Testosterone 6-hydroxylationInitial linearity studies were performed and revealed that this reaction was linear up to 0.2 mg microsomal protein for both species and an incubation time of 15 min in human and 30 min in rat microsomal incubations. A 500 l incubation mixture containing 0.05 mg microsomal protein (human or rat) was incubated with testosterone in the SPHINX31 presence of MgCl2 (10 mm) and NADPH (2.5 mm) in phosphate buffer.