Salen G, Batta A K, Tint G S, Shefer S, Ness G C
VA Medical Center, East Orange, NJ 07018.
J Lipid Res. 1994 Oct;35(10):1878-87.
We investigated the relationship between plasma cholestanol (5 alpha-dihydrocholesterol) concentrations and the activity and mRNA levels of cholesterol 7 alpha-hydroxylase, the rate-controlling enzyme for bile acid synthesis, in three female sitosterolemic homozygotes. In this lipid storage disease, large amounts of plant sterols and cholestanol accumulate because of hyperabsorption and endogenous synthesis, respectively. Plasma cholestanol concentrations were 14 times greater in the three sitosterolemic homozygotes than the mean for five control subjects. To investigate the cholestanol biosynthetic pathway, tracer doses of two putative precursors, [1,2-3H]4-cholesten-3-one and [4-14C]7 alpha-hydroxycholesterol were injected intravenously into a homozygote, and radioactivity was sought in cholestanol, bile acids, cholesterol, and sitosterol fractions isolated from plasma and bile. Tritium was concentrated only in cholestanol; neither cholesterol, sitosterol nor bile acids were derived from [1,2-3H]4-cholesten-3-one. In contrast, bile acids were labeled exclusively with 14C from [4-14C]7 alpha-hydroxycholesterol; no 14C radioactivity was detected in cholestanol. Mathematical analysis of specific activity versus time curves for [3H]cholestanol revealed very slow decay, large exchangeable pools, and enhanced synthesis in the sitosterolemic homozygote. Measurements of cholesterol 7 alpha-hydroxylase activity were 39% lower in whole liver microsomes from three sitosterolemic homozygotes that contained 19% plant sterols as compared to the mean value for six control microsomal specimens that contained 0.1% plant sterols. Removal of the excess plant sterols from the microsomes, in vitro, normalized microsomal cholesterol 7 alpha-hydroxylase activity in the homozygotes but did not affect enzyme activity in the controls. Equal amounts of cholesterol 7 alpha-hydroxylase mRNA were detected in the livers of both control and sitosterolemic subjects. Bile acid malabsorption after ileal bypass surgery stimulated cholesterol 7 alpha-hydroxylase activity 78% in sitosterolemic whole liver microsomes and reduced plasma cholesterol, sitosterol, and cholestanol levels 61%, 55% and 91%, respectively, producing a pronounced decrease in the cholestanol/cholesterol ratio without changing the sitosterol/cholesterol ratio. These results demonstrate that increased cholestanol is synthesized from 4-cholesten-3-one and not 7 alpha-hydroxycholesterol in sitosterolemia. Enhanced pools and plasma concentrations are related inversely to hepatic cholesterol 7 alpha-hydroxylase activity. Competitive inhibition of cholesterol 7 alpha-hydroxylase by the large microsomal plant sterol pool diverts cholesterol into cholestanol. Alternatively, stimulating cholesterol 7 alpha-hydroxylase activity after ileal bypass surgery markedly diminished plasma cholestanol levels.(ABSTRACT TRUNCATED AT 400 WORDS)
我们研究了三名女性纯合子谷甾醇血症患者血浆胆甾烷醇（5α-二氢胆固醇）浓度与胆汁酸合成的限速酶胆固醇7α-羟化酶的活性及mRNA水平之间的关系。在这种脂质贮积病中，由于植物甾醇的过度吸收和胆甾烷醇的内源性合成，大量的植物甾醇和胆甾烷醇会蓄积。三名谷甾醇血症纯合子患者的血浆胆甾烷醇浓度比五名对照受试者的平均值高14倍。为了研究胆甾烷醇的生物合成途径，向一名纯合子静脉注射了两种假定前体的示踪剂量，即[1,2-³H]4-胆甾烯-3-酮和[4-¹⁴C]7α-羟基胆固醇，并在从血浆和胆汁中分离出的胆甾烷醇、胆汁酸、胆固醇和谷甾醇组分中寻找放射性。³H仅在胆甾烷醇中富集；胆固醇、谷甾醇和胆汁酸均非源自[1,2-³H]4-胆甾烯-3-酮。相反，胆汁酸仅被来自[4-¹⁴C]7α-羟基胆固醇的¹⁴C标记；在胆甾烷醇中未检测到¹⁴C放射性。对[³H]胆甾烷醇的比活性与时间曲线进行数学分析显示，其衰变非常缓慢，可交换池很大，且在谷甾醇血症纯合子中合成增强。三名谷甾醇血症纯合子患者的全肝微粒体中胆固醇7α-羟化酶活性比六个含有0.1%植物甾醇的对照微粒体标本的平均值低39%，这些纯合子全肝微粒体中含有19%的植物甾醇。在体外从微粒体中去除过量的植物甾醇后，纯合子的微粒体胆固醇7α-羟化酶活性恢复正常，但对对照的酶活性没有影响。在对照和谷甾醇血症患者的肝脏中检测到等量的胆固醇7α-羟化酶mRNA。回肠旁路手术后胆汁酸吸收不良使谷甾醇血症全肝微粒体中的胆固醇7α-羟化酶活性提高了78%，并使血浆胆固醇、谷甾醇和胆甾烷醇水平分别降低了61%、55%和91%，使胆甾烷醇/胆固醇比值显著降低，而谷甾醇/胆固醇比值不变。这些结果表明，在谷甾醇血症中，胆甾烷醇增加是由4-胆甾烯-3-酮而非7α-羟基胆固醇合成的。胆甾烷醇池和血浆浓度的增加与肝脏胆固醇7α-羟化酶活性呈负相关。大量微粒体植物甾醇池对胆固醇7α-羟化酶的竞争性抑制将胆固醇转向胆甾烷醇的合成。另外，回肠旁路手术后刺激胆固醇7α-羟化酶活性可显著降低血浆胆甾烷醇水平。（摘要截断于400字）
