Esterification makes retinol more labile to photolysis

Because retinyl palmitate was reported to be more stable to oxidation than retinol, we wondered if retinyl palmitate was also more resistant to photolysis as compared to free alcohol. We investigated the resistance of ethanolic solutions of retinol, retinyl palmitate, or both to air oxidation and (or) photolysis using fluorescent light. The initial concentrations were all-trans-retinol, 14 mumol/L, and all-trans-retinyl palmitate, 14 mumol/L. The concentrations of retinol and retinyl palmitate were determined by HPLC and are expressed as a percentage of their original concentrations. After 4 h of exposure to an 18 W fluorescent lamp at 15 cm from the solution, the means (SD) of the surviving analytes were 64% (3%) for retinol and 5% (2%) for retinyl palmitate in a solution containing both retinol and retinyl palmitate. Taking account of the cis isomer arising from retinyl palmitate, 29% (3%) of the retinyl palmitate survived after 4 h of photolysis. Degradation of retinyl palmitate might occur after the conversion of trans isomer to cis isomer during photolysis, however, trans isomer could be degraded with a lesser extent of isomerization. After 4 h of bubbling air through the solution in the dark, 49% (6%) of retinol and 69% (4%) of retinyl palmitate survived. Exposing retinol or retinyl palmitate separately to air oxidation, bubbling air through the solution, or photolysis, exposing them to light, we found that retinyl palmitate could retard the air oxidation of retinol (p < 0.001), but it had no effect on the light-induced degradation of retinol. We also studied the effect of the addition of approximately 1,560 mumol/L alpha-tocopherol, approximately 190 mumol/L beta-carotene and approximately 2,000 mumol/L ascorbic acid as antioxidants. In the presence of 156 mumol/L alpha-tocopherol, 87% (1%) of the retinol and 91% (4%) of the retinyl palmitate remained after air oxidation. Although the photolysis of retinol and retinyl palmitate was also inhibited by 190 mumol/L beta-carotene, alpha-tocopherol and ascorbic acid did not exert inhibiting effects. We conclude that retinyl palmitate is physico-chemically more labile to photolysis but is more resistant to air oxidation than retinol.