PDF(Pubmed)
Abstract：
Capture of a photon by an opsin visual pigment isomerizes its 11-cis-retinaldehyde (11cRAL) chromophore to all-trans-retinaldehyde (atRAL), which subsequently dissociates. To restore light sensitivity, the unliganded apo-opsin combines with another 11cRAL to make a new visual pigment. Two enzyme pathways supply chromophore to photoreceptors. The canonical visual cycle in retinal pigment epithelial cells supplies 11cRAL at low rates. The photic visual cycle in Müller cells supplies cones with 11-cis-retinol (11cROL) chromophore precursor at high rates. Although rods can only use 11cRAL to regenerate rhodopsin, cones can use 11cRAL or 11cROL to regenerate cone visual pigments. We performed a screen in zebrafish retinas and identified ZCRDH as a candidate for the enzyme that converts 11cROL to 11cRAL in cone inner segments. Retinoid analysis of eyes from Zcrdh-mutant zebrafish showed reduced 11cRAL and increased 11cROL levels, suggesting impaired conversion of 11cROL to 11cRAL. By microspectrophotometry, isolated Zcrdh-mutant cones lost the capacity to regenerate visual pigments from 11cROL. ZCRDH therefore possesses all predicted properties of the cone 11cROL dehydrogenase. The human protein most similar to ZCRDH is RDH12. By immunocytochemistry, ZCRDH was abundantly present in cone inner segments, similar to the reported distribution of RDH12. Finally, RDH12 was the only mammalian candidate protein to exhibit 11cROL-oxidase catalytic activity. These observations suggest that RDH12 in mammals is the functional ortholog of ZCRDH, which allows cones, but not rods, to regenerate visual pigments from 11cROL provided by Müller cells. This capacity permits cones to escape competition from rods for visual chromophore in daylight-exposed retinas.
摘要：
视蛋白视觉颜料捕获光子会将其11-顺式视黄醛（11cRAL）发色团异构化为全反式视黄醛（atRAL），随后分离。为了恢复光敏感度，无配体的apo-opsin与另一个11cRAL结合，形成一种新的视觉颜料。两种酶途径为光感受器提供发色团。视网膜色素上皮细胞的典型视觉周期以低速率供应11cRAL。Müller细胞中的光视觉周期以高速率为视锥提供11-顺式视黄醇（11cROL）发色团前体。虽然棒只能使用11cRAL再生视紫红质，锥体可以使用11cRAL或11cROL再生锥体视觉颜料。我们在斑马鱼视网膜中进行了筛选，并将ZCRDH鉴定为将视锥内段中的11cROL转化为11cRAL的候选酶。对Zcrdh突变斑马鱼的眼睛进行的视黄醇分析显示11cRAL水平降低，11cROL水平升高，提示11cROL转化为11cRAL受损。通过显微分光光度法，分离的Zcrdh突变体视锥细胞失去了从11cROL再生视觉色素的能力。因此,ZCRDH具有锥11cROL脱氢酶的所有预测性质。与ZCRDH最相似的人蛋白是RDH12。通过免疫细胞化学，ZCRDH大量存在于圆锥内段，与报告的RDH12分布相似。最后,RDH12是唯一表现出11cROL-氧化酶催化活性的哺乳动物候选蛋白。这些观察结果表明，哺乳动物中的RDH12是ZCRDH的功能直系同源物，允许锥体，但不是杆，从Müller细胞提供的11cROL再生视觉色素。这种能力允许视锥细胞在日光暴露的视网膜中逃避与视杆竞争的视觉发色团。
