Subcellular distribution of zinc and the presence of a metallothionein-like protein in bovine retina.

The mammalian retinas contain the highest concentrations of Zn in any known living tissues. Metallothionein is a low-molecular-weight, cysteine-rich metal-binding protein which occurs ubiquitously in nature and which has been recently identified in mammalian brains with a high affinity to bind Zn. In order to study the metabolism of Zn further, we have measured its subcellular distribution in the bovine retina and have found the distribution to be nonuniform, with the outer rod segments and the pellet 1 fraction, known to be highly enriched in photoreceptor cell synaptosomes, containing the highest amounts of 0.230 +/- 0.040 and 0.119 +/- 0.04 ? Zn/mg protein, respectively. In addition, the bovine retina contains a low-molecular-weight metallothionein-like protein which exhibits an elution volume (V(c)/V(0)) of 1.9 on gel permeation chromatography and which produces only one isoform on reverse-phase high performance liquid chromatography with a retention time of 16.67 min. The precise function of this metallothioneinlike protein, which may be related to the transport and compartmentation of Zn in the retina, remains to be elucidated.