Environmental effects on the autoxidation of retinol

Abstract

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 O2, within minutes of preparation. 3. The maximum rate of O2 uptake is approximately linearly dependent on the concentration of O2. 4. At limiting concentrations of O2, the spectral changes are accelerated by catalase, indicating that H2O2 is one of the reaction products. 5. The autoxidation, which is relatively unaffected by light, has the characteristics of a radical-catalysed reaction. O2 uptake is preceded by an exceptionally short induction period; the reaction is catalysed by Fe2+ 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 O2, the cation-induced aggregates exhibit a spectral red-shift, which difference-spectra indicate is caused by formation of a species with λmax. 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.