Competitive inhibition by 11- cis retinaldehyde of all- trans retinol binding to the individual modules of Xenopus IRBP. Binding of all-trans retinol to the individual modules was measured by fluorescence enhancement in the presence of varying amounts of 11-cis retinal. Panels A through D correspond to modules 1 through 4 respectively. Assuming both all-trans retinol and 11-cis retinal share the same binding sites, all-trans retinol binding to IRBP is competitively inhibited. Three dimensional nonlinear regression was used to determine the number of binding sites, the dissociation constant of all-trans retinol, and the dissociation constant of 11-cis retinal for each of the individual modules. The binding parameters are summarized in Table 3. The concentrations of the proteins used in the titrations were as follows: module 1 (0.99 μM; panel A); module 2 (1.10 μM; panel B); module 3 (1.10 μM; panel C); module 4 (1.00 μM; panel D).