EVIDENCE OF CIS-TRANS ISOMERIZATION OF 9-CIS-BETA-CAROTENE DURING ABSORPTION IN HUMANS

Absorption and metabolism of [C-13]9-cis-beta-carotene ([C-13]9c beta C) was studied in three subjects after a single oral dose. Subjects gi ven 1.0 mg [C-13]beta-carotene (mean: 99.4% 9-cis-beta-carotene, 0.6% all-trans-beta-carotene; dose A) had substantial concentrations of [C- 13]all-trans-beta-carotene ([C-13]tr beta C) and [C-13]all-trans retin ol ([C-13]retinol) but very low concentrations of [C-13]cis-beta-carot ene ([C-13]cis beta C) in saponified plasma 5 h after dosing, as deter mined by HPLC and isotope-ratio mass spectrometry. There was no eviden ce of appreciable absorption of [C-13]9-cis retinol. To determine the proportion of [C-13]tr beta C and [C-13]retinol derived from [C-13]9c beta C, a second set of studies in the same subjects was performed wit h the same isomeric composition except with C-13 labeling only in all- trans-beta-carotene (dose B). The results indicated that > 95% of plas ma [C-13]tr beta C and [C-13]retinol observed after dose A was derived from [C-13]9c beta C. The concentrations of [C-13]tr beta C observed, in excess of that derived from the trace amounts of [C-13]tr beta C i n the dose, indicated that a significant proportion of the [C-13]9c be ta C dose was isomerized to [C-13]tr beta C before entering the bloods tream. Although precise quantitative estimates of the extent of isomer ization of 9-cis-beta-carotene could not be made, it is apparent that cis-trans isomerization of 9-cis-beta-carotene to all-trans-beta-carot ene contributed to the near absence of postprandial plasma 9-cis-beta- carotene after its oral administration in humans. The observation of d ifferent ratios of beta-carotene to retinol between the two dosing pro tocols suggests that isomerization did not occur exclusively before up take by the intestinal mucosa. These results indicate that isomerizati on of ingested 9-cis-beta-carotene before its secretion into the blood stream limits the potential supply of 9-cis retinoids to tissues, and increases the vitamin A value of 9-cis-beta-carotene.