The enzyme lactate dehydrogenase as a structural protein in avian and crocodilian lenses

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Abstract
The major components of mammalian lenses are tissue-specific, soluble proteins, the α-, β- and γ-crystallins1–3. The lenses of other vertebrate classes often contain other major proteins, notably δ-crystallin in birds and reptiles4. A fourth distinct type, described as ε-crystallin, is prominent in many bird and crocodile lenses5,6. Here we show that ε-crystallin is an active glycolytic enzyme, lactate dehydrogenase (LDH) (EC 1.1.1.27) and that duck ε-crystallin appears to be identical to duck LDH-B4. LDH is a normal metabolic component in other lenses7, 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.