Büch K, Stransky H, Hager A
Botanisches Institut, Universität Tübingen, Germany.
FEBS Lett. 1995 Nov 27;376(1-2):45-8. doi: 10.1016/0014-5793(95)01243-9.
In chloroplasts of plants the xanthophyll cycle is suggested to function as a protection mechanism against photodamage. Two enzymes catalyze this cycle. One of them, violaxanthin de-epoxidase, transforms violaxanthin (Vio) to zeaxanthin (Zea) via antheraxanthin (Anth) and is bound to the lumenal surface of the thylakoid vesicles, when being in its active state. The other enzyme, Zea-epoxidase, is responsible for the backward reaction (Zea-->Anth-->Vio) and is active at the stromal side of the thylakoid. For the epoxidation of Zea this enzyme requires NAD(P)H and O2 as cosubstrates. Using isolated thylakoid membranes we found that FAD enhances the epoxidase activity (decrease of apparent Km for NAD(P)H and two-fold increase of Vmax). The flavin functions as a third cofactor which is partially lost during the isolation procedure of thylakoids. Other flavins, such as FMN or riboflavin are without effect. The involvement of FAD in the enzymatic reaction is also demonstrated by the inhibitory action of diphenyleneiodoniumchloride (DPI) (IC50 = 2.3 microM), a compound that blocks the reoxidation of reduced flavins within enzymes. The Zea-epoxidase is a multi-component enzyme system which can be classified as FAD-containing, NAD(P)H- and O2-dependent monooxygenase that is able to epoxidize 3-hydroxy beta-ionone rings of xanthophylls in the 5,6 position.
在植物叶绿体中，叶黄素循环被认为是一种防止光损伤的保护机制。两种酶催化此循环。其中一种，紫黄质脱环氧化酶，在其处于活性状态时，通过花药黄质将紫黄质（Vio）转化为玉米黄质（Zea），并与类囊体小泡的腔表面结合。另一种酶，玉米黄质环氧化酶，负责逆向反应（Zea→花药黄质→紫黄质），并在类囊体的基质侧具有活性。对于玉米黄质的环氧化，该酶需要NAD(P)H和O₂作为共底物。使用分离的类囊体膜，我们发现FAD可增强环氧化酶活性（NAD(P)H的表观Km降低，Vmax增加两倍）。黄素作为第三种辅因子，在类囊体的分离过程中会部分丢失。其他黄素，如FMN或核黄素则无此作用。二苯基碘鎓氯化物（DPI）（IC50 = 2.3 μM）的抑制作用也证明了FAD参与酶促反应，DPI是一种能阻断酶中还原黄素再氧化的化合物。玉米黄质环氧化酶是一种多组分酶系统，可归类为含FAD、依赖NAD(P)H和O₂的单加氧酶，能够将叶黄素的3-羟基β-紫罗兰酮环在5,6位进行环氧化。
