Localization of lactate dehydrogenase activity in the cells of the fish (Xiphophorus helleri) eye

The swordtail (Xiphophorus helleri) eye contains the A₄ and B₄ isozymes of lactate dehydrogenase (LDH) as do all vertebrates and in addition contains a distinctive lactate dehydrogenase, the E₄ isozyme, found only in fish. LDH in the fish eye is located predominantly in the neural retina, especially in the inner segments of the photoreceptor cells. This neural retinal LDH activity consists mainly of the B₄ and E₄ isozymes which are more resistant than the A₄ isozyme to inhibition by 1.8 M urea. The sclera, cornea, and lens possess mainly the urea‐sensitive A₄ isozyme. The specific cellular location of the E₄ isozyme suggests that it plays a role in the biochemistry of vision.