RETINOIC ACID CAN BE PRODUCED FROM EXCENTRIC CLEAVAGE OF BETA-CAROTENE IN HUMAN INTESTINAL-MUCOSA

The hypothesis that retinoic acid (RA) is produced from the excentric cleavage of β-carotene was tested in human intestinal homogenates in vitro. Significant amounts of RA were identified by HPLC and derivatization after incubation of intestinal mucosal homogenates with retinal, β-carotene, or β-apocarotenals at 37 °C for 60 min. RA formation was inhibited, in a dose-dependent fashion, when retinal was incubated in the presence of 0.1-3.0 mm citral (3,7-dimethyl-2,6-octadienal) under identical experimental conditions. The formation of RA from both β-carotene and β-apocarotenals was dose and time dependent and RA was the major metabolite of both β-apo8′-carotenal and β-apo- 12′-carotenal after the incubation. However, citral (0.1 to 4 mm) did not inhibit the formation of β-apocarotenals and RA from 2 μm β-carotene (P > 0.05), which proves the existence of an excentric cleavage mechanism for β-carotene conversion into retinoids. Furthermore, RA formation from both β-apo-8′-carotenal and β-apo-12′-carotenal in human intestinal homogenate occurred in the presence of citral, which demonstrates that RA can be produced from excentric cleavage of β-carotene via a series of β-apocarotenals as intermediates. © 1992.