Environmental effects on the autoxidation of retinol.

1. The behaviour of retinol in aqueous colloidal dispersions has been studied because, if membranes are a physiological site of action of vitamin A, the reactions of colloidal retinol may be relevant to the functions of the vitamin in vivo. 2. Dispersions of retinol in NaCl exhibit characteristic spectral changes, and they consume O(2), within minutes of preparation. 3. The maximum rate of O(2) uptake is approximately linearly dependent on the concentration of O(2). 4. At limiting concentrations of O(2), the spectral changes are accelerated by catalase, indicating that H(2)O(2) is one of the reaction products. 5. The autoxidation, which is relatively unaffected by light, has the characteristics of a radical-catalysed reaction. O(2) uptake is preceded by an exceptionally short induction period; the reaction is catalysed by Fe(2+) ions and is inhibited by diphenylpicrylhydrazyl. 6. The maximum rate of autoxidation, which is less in water or sucrose solution than in saline, depends on the degree of aggregation of retinol molecules induced by cations. 7. In the absence of O(2), the cation-induced aggregates exhibit a spectral red-shift, which difference-spectra indicate is caused by formation of a species with lambda(max.) 370-380nm. 8. This species, from which retinol can be quantitatively recovered, is apparently the oxygen-sensitive form of retinol that initiates the rapid autoxidation. 9. The possible biological significance of the production of a highly reactive form of retinol in micellar aggregates is discussed.