Cholesterol biosynthesis from lanosterol: differential inhibition of sterol delta 8-isomerase and other lanosterol-converting enzymes by tamoxifen.
The fact that administration of tamoxifen (Tam) to humans and laboratory animals (e.g., rats and monkeys) results in both a drastic reduction in cholesterol and a marked accumulation of certain sterol intermediates in their serum led us to undertake more direct biochemical studies on the mechanism of Tam's inhibitory action on the cholesterogenic enzymes. Of the five rat hepatic lanosterol-converting enzymes examined, the enzyme most sensitive to inhibition by Tam was sterol delta 8-isomerase (delta 8-SI) (a 208-fold inhibition relative to lanosterol 14 alpha-methyl demethylase), followed by sterol delta 24-reductase (13-fold) and sterol delta 14-reductase (5.2-fold). The inhibition patterns of all four affected enzymes were found to be noncompetitive, despite widely different inhibition constants (Ki) of 0.21 to 23.5 microM. The inhibitory activity of Tam on delta 8-SI was not affected by detergent-mediated solubilization of the microsomes. In Chinese hamster ovary cells, inhibition of delta 8-SI activity (IC50 = 0.15 microM) was paralleled by a decreased rate of [14C]-mevalonate incorporation into cholesterol (IC50 = 0.70 microM). Our results should provide more insight into an underlying mechanism of Tam's cardioprotective role by interfering the operation of the pathway of cholesterol biosynthesis from lanosterol in mammals.
Cho
S Y
SY
Department of Biochemistry, Yonsei University, Seoul, Korea.
Kim
J H
JH
Paik
Y K
YK
D000818:Animals; D016466:CHO Cells / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D002784:Cholesterol / Q000096:biosynthesis*; D006224:Cricetinae; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004965:Estrogen Antagonists / Q000494:pharmacology / Q000633:toxicity; D005057:Eukaryotic Cells / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism; D007700:Kinetics; D007810:Lanosterol / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008297:Male; D010088:Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism; D044925:Oxidoreductases Acting on CH-CH Group Donors; D051381:Rats; D017207:Rats, Sprague-Dawley; D013251:Steroid Isomerases / Q000037:antagonists & inhibitors / Q000378:metabolism; D058886:Sterol 14-Demethylase; D013629:Tamoxifen / Q000494:pharmacology / Q000633:toxicity
C548418:CYP51A1 protein, human; C548417:Cyp51 protein, rat; D065607:Cytochrome P-450 Enzyme Inhibitors; D004965:Estrogen Antagonists; D013629:Tamoxifen; D007810:Lanosterol; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D010088:Oxidoreductases; D058886:Sterol 14-Demethylase; D044925:Oxidoreductases Acting on CH-CH Group Donors; C022673:delta 24-sterol reductase; C019995:7-dehydrocholesterol reductase; C022672:delta(14)-sterol reductase; D013251:Steroid Isomerases; C022669:delta(8)-delta(7)-sterol isomerase