Miettinen T A
J Lipid Res. 1982 Mar;23(3):466-73.
Animal cholesterol synthesis shows a marked diurnal variation, a phenomenon, at the moment, not known to occur in man. Since cholesterol precursors in serum reflect overall cholesterol synthesis in many conditions, a 24-hr profile of squalene and methyl sterols was studied in plasma lipoproteins in order to demonstrate whether these cholesterol precursors could exhibit a diurnal cycling in healthy human subjects. During the 24-hr period, lipoproteins of density < 1.006 g/ml transported 30-50% of the plasma squalene. Free methyl sterols were found mainly in low density lipoproteins (LDL) and esterified methyl sterols in LDL and high density lipoproteins (HDL). Postprandial hyperlipidemia at noon was associated with an inconsistent increase of the squalene and free methyl sterol concentrations in the lipoproteins of density < 1.006 g/ml. In terms of micro g per mg of cholesterol, the precursor contents were, however, low in each lipoprotein during the daytime. During the night and early morning, the values were several times higher. Thus the peak plasma squalene and methyl sterol contents occurred at midnight and 4 am. The highest variation was found for squalene in the density class < 1.006 g/ml and for lanosterol and diunsaturated dimethyl sterol in LDL and HDL. For different methyl sterols, the mean diurnal variation was 3.5- to 6.9-fold in LDL, 2.0- to 4.5-fold in HDL, and 2.6- to 3.6-fold in the density class < 1.006 g/ml. The respective values for squalene were 2.2, 1.4, and 2.9. Esterified methyl sterols varied slightly in the density class < 1.006 g/ml only, and the percentage esterification exhibited a diurnal fluctuation that was the reciprocal of that of free methyl sterol levels. The rapid and marked diurnal fluctuation of squalene and free methyl sterols in plasma lipoproteins suggests that these precursors are metabolized on and off lipoproteins. The variation is most likely caused by changes in cholesterol synthesis, inferring that circadian rhythm also regulates human cholesterol production.-Miettinen, T. A. Diurnal variation of cholesterol precursors squalene and methyl sterols in human plasma lipoproteins.
动物胆固醇合成呈现出显著的昼夜变化，目前这种现象在人类中尚未发现。由于血清中的胆固醇前体在许多情况下反映了整体胆固醇合成，因此研究了血浆脂蛋白中角鲨烯和甲基固醇的24小时变化情况，以证明这些胆固醇前体在健康人类受试者中是否会呈现昼夜循环。在24小时期间，密度<1.006 g/ml的脂蛋白运输了血浆中30%-50%的角鲨烯。游离甲基固醇主要存在于低密度脂蛋白（LDL）中，而酯化甲基固醇则存在于LDL和高密度脂蛋白（HDL）中。中午的餐后高脂血症与密度<1.006 g/ml的脂蛋白中角鲨烯和游离甲基固醇浓度的不一致增加有关。然而，以每毫克胆固醇中的微克数计算，白天每种脂蛋白中的前体含量都很低。在夜间和清晨，这些值会高出几倍。因此，血浆角鲨烯和甲基固醇含量的峰值出现在午夜和凌晨4点。在密度<1.006 g/ml的类别中，角鲨烯的变化最大，在LDL和HDL中羊毛甾醇和双不饱和二甲基固醇的变化最大。对于不同的甲基固醇，LDL中的平均昼夜变化为3.5至6.9倍，HDL中为2.0至4.5倍，密度<1.006 g/ml的类别中为2.6至3.6倍。角鲨烯的相应值分别为2.2、1.4和2.9。仅在密度<1.006 g/ml的类别中，酯化甲基固醇略有变化，酯化百分比呈现出与游离甲基固醇水平相反的昼夜波动。血浆脂蛋白中角鲨烯和游离甲基固醇的快速且显著的昼夜波动表明，这些前体在脂蛋白上或脂蛋白外进行代谢。这种变化很可能是由胆固醇合成的变化引起的，这意味着昼夜节律也调节人类胆固醇的产生。-米耶蒂宁，T.A. 人血浆脂蛋白中胆固醇前体角鲨烯和甲基固醇的昼夜变化。
