WALD G, HUBBARD R
J Gen Physiol. 1949 Jan;32(3):367-89. doi: 10.1085/jgp.32.3.367.
In the surviving vertebrate retina the retinene(1) liberated by bleaching rhodopsin is converted quantitatively to vitamin A(1). Recent chemical studies have indicated that in this process the aldehyde group of retinene(1) is reduced to the primary alcohol group of vitamin A(1) (Morton; Wald). Some time ago we brought this reaction into a cell-free brei prepared from cattle retinas. The retinas were frozen, desiccated, ground, and exhaustively extracted with petroleum ether; the resulting powder, stirred in neutral buffer solution and exposed to light, converted its retinene(1) completely to vitamin A(1). Some time ago also we observed that fresh rhodopsin solutions exhibit a special type of fading in darkness following exposure to light, which is absent from the same solutions after aging. We have confirmed Bliss's identification of this reaction as the conversion of retinene(1) to vitamin A(1). The system which reduces retinene(1) is fractionated anatomically in the retinal rods. The outer segments of the rods, broken off from the underlying retinal tissue, are unable to convert their retinene(1) to vitamin A(1). In the presence of a water extract of crushed retina they do perform this conversion. On the other hand the retinal tissue from which a water extract was taken has lost this capacity. Such washed retinal tissue is reactivated by returning the washings to the solid material. The activating effect of retinal washings on isolated outer limbs or washed retina is duplicated by a boiled muscle juice. This in turn can be replaced by reduced cozymase (reduced coenzyme I; DPN-H(2)); or by a mixture of DPN and fructosediphosphate. The conversion of retinene(1) to vitamin A(1) is therefore a reduction in which two atoms of hydrogen are transferred to retinene(1) from reduced cozymase. It is assumed that this reaction is catalyzed by an apoenzyme, retinene(1) reductase, present in the rod outer limb. This process is coupled with a second system in the outer segment which reduces DPN, using hexosediphosphate or one of its derivatives as hydrogen donor. This action of DPN brings a member of the vitamin B complex, nicotinic acid amide, into an auxiliary position in the rhodopsin system. In the isolated retina or in vitro systems the reduction of retinene(1) proceeds irreversibly. Yet this reduction must be balanced by an oxidative process elsewhere in the rhodopsin cycle, since through rhodopsin as intermediate vitamin A(1) regenerates retinene(1).
在存活的脊椎动物视网膜中，视紫红质漂白所释放的视黄醛（1）定量地转化为维生素A（1）。最近的化学研究表明，在这个过程中，视黄醛（1）的醛基被还原为维生素A（1）的伯醇基（莫顿；瓦尔德）。不久前，我们将这个反应引入到由牛视网膜制备的无细胞匀浆中。将视网膜冷冻、干燥、研磨，并用石油醚彻底提取；将所得粉末在中性缓冲溶液中搅拌并暴露于光下，其视黄醛（1）完全转化为维生素A（1）。不久前我们还观察到，新鲜的视紫红质溶液在光照后于黑暗中呈现出一种特殊类型的褪色，而老化后的相同溶液则没有这种现象。我们已经证实布利斯将此反应鉴定为视黄醛（1）向维生素A（1）的转化。还原视黄醛（1）的系统在视网膜杆中按解剖结构分级。从下面的视网膜组织分离出来的杆的外段，无法将其视黄醛（1）转化为维生素A（1）。在存在破碎视网膜的水提取物的情况下，它们确实能进行这种转化。另一方面，从中提取了水提取物的视网膜组织已经失去了这种能力。通过将洗涤液重新加入固体物质中，这种洗涤过的视网膜组织会被重新激活。视网膜洗涤液对分离的外段或洗涤过的视网膜的激活作用可由煮沸的肌肉汁复制。反过来，这又可以由还原型辅酶（还原型辅酶I；DPN - H₂）或由DPN和果糖二磷酸的混合物替代。因此，视黄醛（1）向维生素A（1）的转化是一种还原反应，其中两个氢原子从还原型辅酶转移到视黄醛（1）上。据推测，这个反应由存在于杆外段的一种脱辅酶，即视黄醛（1）还原酶催化。这个过程与外段中的第二个系统相偶联，该系统利用己糖二磷酸或其衍生物之一作为氢供体来还原DPN。DPN的这种作用使维生素B族的一员，烟酰胺，在视紫红质系统中处于辅助地位。在分离的视网膜或体外系统中，视黄醛（1）的还原是不可逆的。然而，这种还原必须由视紫红质循环中其他地方的氧化过程来平衡，因为通过视紫红质作为中间体，维生素A（1）再生视黄醛（1）。
