The metabolism of 17 alpha‐ethinyloestradiol by human liver microsomes: formation of catechol and chemically reactive metabolites.

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1. The metabolism of amodiaquine (A), ethinyloestradiol (E), mianserin (M), phenytoin (Ph), sulphanilamide (S) and paracetamol (Pa) to both stable and chemically reactive, i.e. irreversibly protein bound, metabolites was investigated using microsomes prepared from histologically normal human liver obtained from eight kidney donors. 2. All drugs, except amodiaquine, were metabolized by NADPH‐dependent microsomal enzymes to chemically reactive metabolites. The degree of NADPH‐dependent binding varied between drugs (E, 11.5 +/‐ 5.8% incubated drug; M, 3.0 +/‐ 1.9%; Ph, 0.10 +/‐ 0.09%; S, 0.57 +/‐ 0.38%; Pa, 1.2 +/‐ 1.2%; mean of eight livers +/‐ s.d.). 3. Inclusion of glutathione (1 mM) or ascorbic acid (1 mM) in the incubation reduced the NADPH‐dependent binding for all substrates, indicating the involvement of electrophilic oxidation products. 4. Binding of M and Pa correlated with each other (Spearman's r = 0.86) and with total cytochrome P‐450 content (r = 0.76 and 0.78 respectively). E binding also correlated with the binding of M (r = 0.79) and Pa (r = 0.81) but not with cytochrome P‐450. Binding of Ph and S did not correlate with any of the other measured metabolic parameters.

Section: Resultsmentioning

confidence: 61%

A comparative study of the formation of chemically reactive drug metabolites by human liver microsomes.

Kitteringham¹,

Lambert²,

Maggs³

et al. 1988

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“…Livers from donors were cooled to 00 C, divided into 10-20 g portions and frozen in liquid nitrogen within 1 h of removal and stored at -80°C until required for use. Washed human liver microsomes were prepared as reported previously (Purba et al, 1987). Six male CBA/ca mice (25-50 g) were administered phenobarbitone (PB) once daily for 3 days (60 mg kg-') by i.p.…”

Section: Preparation Of Hepatic Microsomesmentioning

confidence: 99%

Direct and metabolism‐dependent toxicity of sulphasalazine and its principal metabolites towards human erythrocytes and leucocytes.

Pirmohamed

Coleman

Hussain

et al. 1991

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6.8 ± 0.3%), the retention time of which on h.p.l.c. corresponded to synthetic sulphapyridine hydroxylamine. The half-life of sulphapyridine hydroxylamine in phosphate buffer (pH 7.4) was found to be 8.1 min. 5 In the absence of microsomes and NADPH, sulphapyridine hydroxylamine caused a concentration-dependent (10-500 FM) increase in methaemoglobinaemia (2.9%-24.4%) and cytotoxicity (5.4%-51.4%), whereas sulphasalazine, sulphapyridine, 5-hydroxy sulphapyridine and 5-aminosalicylic acid had no effect.

“…Ethinyloestradiol 2-hydroxylase activity Incubations containing ethinyloestradiol (25 FM), ascorbic acid (1.0 mM), NADPH (0.6 mM), microsomal protein (4 mg) and 1/15M phosphate buffer (pH 7.4; total volume 3 ml) were performed in 25 ml Erlenmeyer flasks at 37°C with vigorous agitation (Purba et al, 1987). The reaction was stopped after 5 min by extracting with ether (5 ml x 2).…”

Section: Introductionmentioning

confidence: 99%

“…In addition, we have screened a number of drugs for their ability to inhibit tolbutamide 4-hydroxylase in order to elucidate the selectivity of the cytochrome P-450 isozyme(s) catalysing this reaction. (Purba et al, 1987). Cytochrome P-450 was assayed by the method of Omura & Sato (1964) NADPH (lmM), microsomal protein (1.5-2.0 mg) and 1/15M phosphate buffer pH 7.5 (total volume 2.5 ml) were performed in Erlenmeyer flasks at 37°C with vigorous agitation.…”

Section: Introductionmentioning

confidence: 99%

Tolbutamide 4‐hydroxylase activity of human liver microsomes: effect of inhibitors.

Purba

Back

Orme

1987

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Eight samples of human liver have been characterised for microsomal protein content, cytochrome P450 content, tolbutamide 4-hydroxylase and ethinyloestradiol 2-hydroxylase activities. Cytochrome P-450 content correlated significantly with ethinyloestradiol 2-hydroxylase activity but not with tolbutamide 4-hydroxylase activity. There was no significant correlation between ethinyloestradiol 2-hydroxylase and tolbutamide 4-hydroxylase activities. The maximum tolbutamide 4-hydroxylase activity was 0.45 nmol min-' mg-1 microsomal protein, with a Km value of 74 FLM. A number of compounds were tested for their ability to inhibit tolbutamide metabolism. All the compounds showing inhibition were either non-competitive or mixed non-competitive inhibitors of tolbutamide 4-hydroxylation. These studies suggest that tolbutamide is metabolised by an isozyme of cytochrome P-450 which appears to be distinct from those isozymes metabolising many other drugs.