Harwood H J, Greene Y J, Stacpoole P W
J Biol Chem. 1986 Jun 5;261(16):7127-35.
3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity in microsomes isolated from cultured lymphoid (IM-9) cells or freshly isolated human leukocytes was markedly decreased by either ascorbic acid or its oxidized derivative, dehydroascorbate. Inhibition of IM-9 leukocyte HMG-CoA reductase activity was log linear between 0.01 and 10 mM ascorbic acid (25 and 81% inhibition, respectively) and 0.1 and 10 mM dehydroascorbate (5 and 75% inhibition, respectively). Inhibition was noncompetitive with respect to HMG-CoA (Km = 10.2 microM (RS); ascorbic acid, Ki = 6.4 mM; dehydroascorbate, Ki = 15 mM) and competitive with respect to NADPH (Km = 16.3 microM; acetic acid, Ki = 6.3 mM; dehydroascorbate, Ki = 3.1 mM). Ascorbic acid and dehydroascorbate are interconverted through the free radical intermediate monodehydroascorbate. Reducing agents are required to convert dehydroascorbate to monodehydroascorbate, but prevent formation of the free radical from ascorbate. In microsomes from IM-9 cells, the reducing agent, dithiothreitol, abolished HMG-CoA reductase inhibition by ascorbate but enhanced inhibition by dehydroascorbate. In addition, the concentration of monodehydroascorbate present in ascorbate solutions was directly proportional to the degree of HMG-CoA reductase inhibition by 1.0 mM ascorbate. Fifty per cent inhibition of enzyme activity occurred at a monodehydroascorbate concentration of 14 microM. These data indicate that monodehydroascorbate mediates inhibition of HMG-CoA reductase by both ascorbate and dehydroascorbate. This effect does not appear to be due to free radical-induced membrane lipid modification, however, since both ascorbate and dehydroascorbate inhibited the protease-solubilized, partially purified human liver enzyme. Since inhibition of HMG-CoA reductase occurs at physiological concentrations of ascorbic acid in the human leukocyte (0.2-1.72 mM), this vitamin may be important in the regulation of endogenous cholesterol synthesis in man.
从培养的淋巴细胞（IM-9）或新鲜分离的人白细胞中分离出的微粒体中的3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶活性，被抗坏血酸或其氧化衍生物脱氢抗坏血酸显著降低。在0.01至10 mM抗坏血酸（分别抑制25%和81%）和0.1至10 mM脱氢抗坏血酸（分别抑制5%和75%）之间，IM-9白细胞HMG-CoA还原酶活性的抑制呈对数线性关系。这种抑制对于HMG-CoA是非竞争性的（Km = 10.2 microM（RS）；抗坏血酸，Ki = 6.4 mM；脱氢抗坏血酸，Ki = 15 mM），而对于NADPH是竞争性的（Km = 16.3 microM；抗坏血酸，Ki = 6.3 mM；脱氢抗坏血酸，Ki = 3.1 mM）。抗坏血酸和脱氢抗坏血酸通过自由基中间体单脱氢抗坏血酸相互转化。需要还原剂将脱氢抗坏血酸转化为单脱氢抗坏血酸，但可防止抗坏血酸形成自由基。在IM-9细胞的微粒体中，还原剂二硫苏糖醇消除了抗坏血酸对HMG-CoA还原酶的抑制，但增强了脱氢抗坏血酸的抑制作用。此外，抗坏血酸溶液中存在的单脱氢抗坏血酸浓度与1.0 mM抗坏血酸对HMG-CoA还原酶的抑制程度成正比。在单脱氢抗坏血酸浓度为14 microM时，酶活性受到50%的抑制。这些数据表明，单脱氢抗坏血酸介导了抗坏血酸和脱氢抗坏血酸对HMG-CoA还原酶的抑制作用。然而，这种作用似乎不是由于自由基诱导的膜脂质修饰，因为抗坏血酸和脱氢抗坏血酸都抑制了蛋白酶溶解的、部分纯化的人肝酶。由于在人白细胞中抗坏血酸的生理浓度（0.2 - 1.72 mM）下会发生HMG-CoA还原酶的抑制，这种维生素可能在人体内源性胆固醇合成的调节中起重要作用。
