Webb T J, Powls R, Rees H H
Department of Biochemistry, University of Liverpool, U.K.
Biochem J. 1995 Dec 1;312 ( Pt 2)(Pt 2):561-8. doi: 10.1042/bj3120561.
In the midgut cytosol of Lepidoptera, ecdysteroids undergo inactivation by transformation via the 3-dehydro derivative to the corresponding 3-epiecdysteroid (3 alpha-hydroxy) and by phosphate conjugation. The oxygen-dependent oxidase catalyses formation of 3-dehydroecdysteroid, which can be reduced either irreversibly by 3-dehydroecdysone 3 alpha-reductase to 3-epiecdysteroid, or by 3-dehydroecdysone 3 beta-reductase back to the initial ecdysteroid. Furthermore, these ecdysteroids undergo further inactivation by phosphorylation. These ecdysteroid transformations have been investigated in last instar larvae of the cotton leafworm, Spodoptera littoralis. The products of the phosphorylation have been characterized as predominantly ecdysteroid 2-phosphate accompanied by smaller amounts of the corresponding 22-phosphate. The phosphotransferases require Mg2+ and ATP. Whereas the 3-dehydroecdysone 3 alpha-reductase has a clear preference for NADPH rather than NADH, the corresponding 3 beta-reductase markedly favours NADH. The physiological significance of the latter enzyme is unclear. The profiles of the various enzymic activities in dialysed midgut cytosol supplemented with appropriate cofactors were determined throughout the last larval instar. All activities were detectable throughout the instar, but the respective enzymes exhibited maxima at different times. Ecdysone oxidase showed a peak early in the instar, with 3-dehydroecdysone 3 alpha-reductase increasing to a peak as the former activity declined. The 3-dehydroecdysone 3 beta-reductase exhibited peak activity late in the instar, a profile similar to that observed for the corresponding haemolymph enzyme involved in reduction of the 3-dehydroecdysone product of the prothoracic glands to ecdysone. Thus, the significance of the midgut 3 beta-reductase may be related to production of active hormone. Both ecydsteroid 22- and 2-phosphotransferases showed high activities early in the instar and then declined. The physiological significance of the profiles for the ecdysone oxidase, the 3-dehydroecdysone 3 alpha-reductase and phosphotransferases is unclear.
在鳞翅目昆虫的中肠胞质溶胶中，蜕皮甾类通过转化为3 -脱氢衍生物形成相应的3 -表蜕皮甾类（3α -羟基）以及通过磷酸化作用而失活。氧依赖性氧化酶催化3 -脱氢蜕皮甾类的形成，其可被3 -脱氢蜕皮激素3α -还原酶不可逆地还原为3 -表蜕皮甾类，或被3 -脱氢蜕皮激素3β -还原酶还原回初始的蜕皮甾类。此外，这些蜕皮甾类通过磷酸化进一步失活。已在棉铃虫（Spodoptera littoralis）的末龄幼虫中研究了这些蜕皮甾类的转化。磷酸化产物主要被鉴定为蜕皮甾类2 -磷酸，并伴有少量相应的22 -磷酸。磷酸转移酶需要Mg2 +和ATP。虽然3 -脱氢蜕皮激素3α -还原酶明显偏好NADPH而非NADH，但相应的3β -还原酶则明显偏好NADH。后一种酶的生理意义尚不清楚。在整个末龄幼虫期测定了添加适当辅因子的透析中肠胞质溶胶中各种酶活性的概况。在整个龄期都可检测到所有活性，但各自的酶在不同时间表现出活性最大值。蜕皮激素氧化酶在龄期早期出现峰值，随着前者活性下降，3 -脱氢蜕皮激素3α -还原酶增加至峰值。3 -脱氢蜕皮激素3β -还原酶在龄期后期表现出峰值活性，这一概况与前胸腺中参与将3 -脱氢蜕皮激素产物还原为蜕皮激素的相应血淋巴酶所观察到的相似。因此，中肠3β -还原酶的意义可能与活性激素的产生有关。蜕皮甾类22 -和2 -磷酸转移酶在龄期早期均表现出高活性，然后下降。蜕皮激素氧化酶、3 -脱氢蜕皮激素3α -还原酶和磷酸转移酶活性概况的生理意义尚不清楚。
