The Enhancement of Fluorescence and the Decreased Susceptibility to Enzymatic Oxidation of Retinol Complexed with Bovine Serum Albumin, fl-Lactoglobulin, and the Retinol-binding Protein of Human Plasma*

The fluorescence yield of bound retinol in human retinolbinding protein (RBP) and irs prealbumin complex (PARBP) were found to be nearly the same and to exceed that of free retinol in petroleum ether by about 9-fold. Bovine serum albumin and P-lactoglobulin also formed strongly fluorescent water-soluble complexes with retinol. The fluorescence yield of retinol complexed with bovine serum albumin was about 4-fold and with P-lactoglobulin about 3-fold larger than retinol in petroleum ether. In the retinol-bovine serum albumin complex the molar ratio of retinol to protein was found to be approximately 1. The greater degree of fluorescence enhancement, the greater stability of retinol in RBP and PA-RBP to spontaneous degradation at room temperature, the relative resistance to extraction of the bound retinol by ether, and the complete unavailability of the retinol in RBP and PA-RBP for oxidation by liver alcohol dehydrogenase, in contrast to the retinol in complexes formed with either bovine serum albumin or P-lactoglobulin, were interpreted as indicating that retinol is bound with greater affinity to the natural carrier protein than it is to bovine serum albumin or /?-lactoglobulin. Retinol was extracted from dry preparations of RBP and PA-RBP with ethanol in the cold, yielding apoprotein preparations that were able to rebind retinol, regenerating about 90 % of their original fluorescence with the bound retinol becoming again completely unreactive toward liver alcohol dehydrogenase.