Evidence of cis-trans isomerization of 9-cis-β-carotene during absorption in humans

Abstract

Absorption and metabolism of [13C]9-cis-β-carotene ([13C]9cβC) was studied in three subjects after a single oral dose. Subjects given 1.0 mg [13C]β-carotene (mean: 99.4% 9-cis-β-carotene, 0.6% all-trans-β- carotene; dose A) had substantial concentrations of [13C]all-trans-β- carotene ([13C]trβC) and [13C]all-trans retinol ([13C]retinol) but very low concentrations of [13C]cis-β-carotene ([13C]cisβC) in saponified plasma 5 h after dosing, as determined by HPLC and isotope-ratio mass spectrometry. There was no evidence of appreciable absorption of [13C]9-cis retinol. To determine the proportion of [13C]trβC and [13C]retinol derived from [13C]9cβC, a second set of studies in the same subjects was performed with the same isomeric composition except with 13C labeling only in all-trans-β-carotene (dose B). The results indicated that > 95% of plasma [13C]trβC and [13C]retinol observed after dose A was derived from [13C]9cβC. The concentrations of [13C]trβC observed, in excess of that derived from the trace amounts of [13C]trβC in the dose, indicated that a significant proportion of the [13C]9cβC dose was isomerized to [13C]trβC before entering the bloodstream. Although precise quantitative estimates of the extent of isomerization of 9-cis-β-carotene could not be made, it is apparent that cis-trans isomerization of 9-cis-β- carotene to all-trans-β-carotene contributed to the near absence of postprandial plasma 9-cis-β-carotene after its oral administration in humans. The observation of different ratios of β-carotene to retinol between the two dosing protocols suggests that isomerization did not occur exclusively before uptake by the intestinal mucosa. These results indicate that isomerization of ingested 9-cis-β-carotene before its secretion into the bloodstream limits the potential supply of 9-cis retinoids to tissues, and increases the vitamin A value of 9-cis-β-carotene.