Transmembrane glucose carriers in the monkey lens. Quantitation and regional distribution as determined by cytochalasin B binding to lens membranes

As yet few of the lens membrane transport proteins have been identified and characterized. The present experiments were therefore undertaken to estimate the number and distribution of glucose transporters in the lens as an initial step to further biochemical characterization of this membrane transport protein. Organ-cultured Rhesus monkey lenses exhibited stereospecific glucose uptake in dual-label experiments using [³H]-L-glucose and [¹⁴h]-3-O-methyl-D-glucose (a non-metabolized D-glucose analogue). Specific D-glucose uptake was inhibited by phloretin and cytochalasin B but was not affected by phloridzin or cytochalasin E. These experiments demonstrated that the monkey lens transport system shares similar properties with those of lenses from previously studied species. In equilibrium binding assays, cytochalasin B bound to a single class of binding sites having a K(D) of 1.02 x 10^-7 M and a B(max) of 71 pmol mg^-1 membrane protein. Urea excretion of lens water-insoluble material resulted in an increase in B(max) to 251 pmol cytochalasin B binding sites mg^-1 membrane protein. Although cytochalasin E, phloridzin and L-glucose had little effect on cytochalasin B binding, both D-glucose and phloretin blocked binding by up to 90%. These findings suggest that, as in a number of other tissues, D-glucose-inhibitable cytochalasin B binding may be used to estimate the number of glucose transporters in the lens. Cytochalasin B binding was measured in membranes isolated from different regions of the lens. Unexpectedly, D-glucose-inhibitable binding activity could be demonstrated throughout the entire lens with the highest specific activity in the nucleus and the lowest in the capsule-epithelium. This distribution is discussed in terms of lens development, metabolic cooperation and function of lens transport proteins.