An hypothesis for a vitamin A cycle in the pigment epithelium of bovine retina.
Several metabolic transformations of vitamin A have now been found in the pigment epithelium of cattle retina, leading to the hypothesis that there may be a vitamin A cycle in these cells. A retinol binding protein of MW ? 17,000 in pigment epithelial cytosol has been identified independently in several laboratories. In addition, a very active esterifying system, capable of utilizing either (3)H-retinol or (3)H-retinol bound to cellular retinol binding protein as substrates, has been demonstrated in the microsomal fraction of the cell. Finally, a retinyl ester hydrolase, with optimum activity between pH 4.0-4.5, utilizing physiological substrate prepared from pigment epithelial microsomes, has been localized in the lysosomal fraction of pigment epithelial cells. When considered together, these transformations could provide a unique cyclic mechanism for storage and mobilization of vitamin A in the pigment epithelium.
Berman
E R
ER
Ophthalmic Biochemistry Unit, Hadassah University Hospital, Jerusalem, Israel.
Segal
N
N
Schneider
A
A
Feeney
L
L