Retinal insulin receptors. 1. Structural heterogeneity and functional characterization

Neural cells of the bovine retina contain specific, high-affinity receptors for insulin. When solubilized and wheat-germ purified, these receptors exhibit a kinase activity that is capable of phosphorylating the receptor's beta-subunit (autophosphorylation) and a tyrosine-containing exogenous substrate, poly (Glu, Tyr) 4:1. Studies of the structure of retinal insulin receptors revealed the existence of two insulin receptor subpopulations. For these populations, the apparent molecular weights of the alpha-subunit were 120- and 133 kDa. This structural heterogeneity does not appear to be related to the presence of vascular contamination and stands in contrast to the brain and liver where a single alpha-subunit type was found (120 kDa for brain and 133 kDa for liver). In addition to being distinguishable by their molecular weights, the two populations of retinal insulin receptors could be distinguished in terms of (a) their solubility in Triton X-100, (b) glycosylation, and (c) recognition by anti-insulin receptor antibody. Despite these structural differences, the two populations of retinal insulin receptors appear to have similar insulin binding affinities.