Wistow G J, Mulders J W, de Jong W W
Nature. 1987;326(6113):622-4. doi: 10.1038/326622a0.
The major components of mammalian lenses are tissue-specific, soluble proteins, the alpha-, beta- and gamma-crystallins. The lenses of other vertebrate classes often contain other major proteins, notably delta-crystallin in birds and reptiles. A fourth distinct type, described as epsilon-crystallin, is prominent in many bird and crocodile lenses. Here we show that epsilon-crystallin is an active glycolytic enzyme, lactate dehydrogenase (LDH) (EC 1.1.1.27) and that duck epsilon-crystallin appears to be identical to duck LDH-B4. LDH is a normal metabolic component in other lenses, but in duck is present in amounts far exceeding the requirements of any likely catalytic role. It appears that an active enzyme has been recruited, unchanged, to an extra role as a structural protein in the lens without gene duplication and sequence divergence. This surprising discovery raises the possibility that other crystallins may similarly be enzymes expressed at high levels in lens as structural proteins.
哺乳动物晶状体的主要成分是组织特异性的可溶性蛋白质，即α-、β-和γ-晶状体蛋白。其他脊椎动物类群的晶状体通常含有其他主要蛋白质，特别是鸟类和爬行动物中的δ-晶状体蛋白。在许多鸟类和鳄鱼的晶状体中，一种被称为ε-晶状体蛋白的第四种独特类型很突出。我们在此表明，ε-晶状体蛋白是一种活性糖酵解酶，即乳酸脱氢酶（LDH）（EC 1.1.1.27），并且鸭ε-晶状体蛋白似乎与鸭LDH-B4相同。LDH是其他晶状体中的正常代谢成分，但在鸭晶状体中的含量远远超过任何可能的催化作用所需。似乎一种活性酶未经基因复制和序列分化就被招募来额外充当晶状体中的结构蛋白。这一惊人发现增加了其他晶状体蛋白可能同样是在晶状体中作为结构蛋白高水平表达的酶的可能性。
