Chappell J., Wolf F., Proulx J., Cuellar R., Saunders C.
Plant Physiology/Biochemistry/Molecular Biology Program, Agronomy Department, University of Kentucky, Lexington,Kentucky 40546-0091 (J.C.).
Plant Physiol. 1995 Dec;109(4):1337-1343. doi: 10.1104/pp.109.4.1337.
3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) catalyzes the irreversible conversion of 3-hydroxy-3-methylglutaryl coenzyme A to mevalonate and is considered a key regulatory step controlling isoprenoid metabolism in mammals and fungi. The rate-limiting nature of this enzyme for isoprenoid biosynthesis in plants remains controversial. To investigate whether HMGR activity could be limiting in plants, we introduced a constitutively expressing hamster HMGR gene into tabacco (Nicotiana tabaccum L.) plants to obtain unregulated HMGR activity. The impact of the resulting enzyme activity on the biosynthesis and accumulation of particular isoprenoids was evaluated. Expression of the hamster HMGR gene led to a 3- to 6-fold increase in the total HMGR enzyme activity. Total sterol accumulation was consequently increased 3- to 10-fold, whereas end-product sterols such as sitosterol, campesterol, and stigmasterol were increased only 2-fold. The level of cycloartenol, a sterol biosynthetic intermediate, was increased more than 100-fold. Although the synthesis of total sterols appears to be limited normally by HMGR activity, these results indicate that the activity of one or more later enzyme(s) in the pathway must also be involved in determining the relative accumulation of end-product sterols. The levels of other isoprenoids such as carotenoids, phytol chain of chlorophyll, and sesquiterpene phytoalexins were relatively unaltered in the transgenic plants. It appears from these results that compartmentation, channeling, or other rate-determining enzymes operate to control the accumulation of these other isoprenoid end products.
3-羟基-3-甲基戊二酰辅酶A还原酶（HMGR）催化3-羟基-3-甲基戊二酰辅酶A不可逆地转化为甲羟戊酸，被认为是控制哺乳动物和真菌类异戊二烯代谢的关键调控步骤。该酶在植物类异戊二烯生物合成中的限速性质仍存在争议。为了研究HMGR活性在植物中是否会成为限制因素，我们将一个组成型表达的仓鼠HMGR基因导入烟草（Nicotiana tabaccum L.）植株中，以获得不受调控的HMGR活性。评估了由此产生的酶活性对特定类异戊二烯生物合成和积累的影响。仓鼠HMGR基因的表达导致总HMGR酶活性增加了3至6倍。总甾醇积累量因此增加了3至10倍，而诸如谷甾醇、菜油甾醇和豆甾醇等终产物甾醇仅增加了2倍。甾醇生物合成中间体环阿屯醇的水平增加了100多倍。尽管总甾醇的合成通常似乎受HMGR活性限制，但这些结果表明该途径中一种或多种后续酶的活性也必定参与决定终产物甾醇的相对积累。在转基因植株中，其他类异戊二烯如类胡萝卜素、叶绿素的植醇链和倍半萜植保素的水平相对未改变。从这些结果看来，区室化、通道化或其他限速酶发挥作用以控制这些其他类异戊二烯终产物 的积累。
