Inhibition by chlorogenic acid of haematin-catalysed retinoic acid 5,6-epoxidation

Chlorogenic acid (3-O-caffeoylquinic acid) inhibited haematin- and haemoglobin-catalysed retinoic acid 5,6-epoxidation. Some other phenol compounds (caffeic acid and 4-hydroxy-3-methoxybenzoic acid) also showed inhibitory effects on the haematin- and haemoglobin-catalysed epoxidation, but salicylic acid did not. Of the above compounds, caffeic acid and chlorogenic acid were potent inhibitors compared with the other two, suggesting that the o-hydroquinone moiety of chlorogenic acid and caffeic acid is essential to the inhibition of the epoxidation. Although caffeic acid inhibited retinoic acid 5,6-epoxidation requiring the consumption of O2, formation of retinoic acid radicals was not inhibited on the addition of caffeic acid to the incubation mixture. The above results suggest that caffeic acid does not inhibit the formation of retinoic acid radicals but does inhibit the step of conversion of retinoic acid radical into the 5,6-epoxide.