Maxwell C A, Edwards R, Dixon R A
Plant Biology Division, Samuel Roberts Noble Foundation, Ardmore, Oklahoma 73402.
Arch Biochem Biophys. 1992 Feb 14;293(1):158-66. doi: 10.1016/0003-9861(92)90379-b.
An O-methyltransferase (OMT) which methylates the 2'-hydroxyl of isoliquiritigenin (2',4,4'-trihydroxychalcone) was identified in alfalfa (Medicago sativa L.) seedlings and cell cultures. The OMT activity increased during early stages of seedling development and was predominantly located in roots. Treatment of alfalfa cell cultures with an elicitor from yeast resulted in a fivefold increase in chalcone OMT activity, whereas treatment of seedlings with CuCl2 caused a reduction in activity. The chalcone OMT was purified to near homogeneity from elicited alfalfa cell cultures. Only one form of the enzyme was found. It consisted of an active monomer of subunit Mr 43,000 which could be photoaffinity labeled with S-adenosyl-L-[methyl-3H]methionine. The purified OMT had a pH optimum of 9.0, pI of 4.7, and was highly specific for the 2'-hydroxyl of 2',4,4'-trihydroxychalcone, with essentially no activity toward narigenin chalcone, caffeic acid, or daidzein. Kinetic analysis indicated a sequential bi bi mechanism with Km values of 2.2 and 17.7 microM for 2',4,4'-trihydroxychalcone and S-adenosyl-L-methionine, respectively. S-Adenosyl-L-homocysteine was a potent inhibitor. The chalcone OMT represents the third distinct OMT isolated from alfalfa cell cultures.
在紫花苜蓿（Medicago sativa L.）幼苗和细胞培养物中鉴定出一种对异甘草素（2',4,4'-三羟基查耳酮）的2'-羟基进行甲基化的O-甲基转移酶（OMT）。OMT活性在幼苗发育早期增加，且主要位于根部。用酵母激发子处理紫花苜蓿细胞培养物导致查耳酮OMT活性增加了五倍，而用CuCl2处理幼苗则导致活性降低。从经激发的紫花苜蓿细胞培养物中纯化得到的查耳酮OMT纯度接近均一。仅发现一种形式的该酶。它由一个分子量为43,000的活性单体组成，该单体可用S-腺苷-L-[甲基-3H]甲硫氨酸进行光亲和标记。纯化的OMT的最适pH为9.0，pI为4.7，对2',4,4'-三羟基查耳酮的2'-羟基具有高度特异性，对柚皮苷查耳酮、咖啡酸或大豆苷元基本无活性。动力学分析表明其为有序双双机制，对2',4,4'-三羟基查耳酮和S-腺苷-L-甲硫氨酸的Km值分别为2.2和17.7 microM。S-腺苷-L-高半胱氨酸是一种强效抑制剂。查耳酮OMT是从紫花苜蓿细胞培养物中分离出的第三种不同的OMT。
