Localization of gamma glutamyl transpeptidase in bovine retina

Gamma-glutamyl transpeptidase (GGTP) is a membrane bound enzyme which has an important role in regulation of glutathione and glutamate in the retina. We have used histochemical and colorimetric enzyme assays to localize GGTP in the bovine retina and choroid. Our results demonstrate that (i) GGTP is present in retinal microvessels but not choroidal microvessels. (ii) Retinal microvascular endothelium loses the ability to express GGTP in cultured cells. (iii) GGTP is present in Muller cells. (iv) Isolated and purified rod outer segments contain high levels of GGTP. (v) Retinal pigment epithelial cells (RPE) in vivo and in culture contain GGTP. The findings of this study lend support to the concept that GGTP may be a biochemical marker for cellular systems which are part of specialized diffusion barriers.