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

• Published in: The American journal of clinical nutrition 1996-08, Vol.64 (2), p.177-183
• Main Authors: You, CS, Parker, RS, Goodman, KJ, Swanson, JE, Corso, TN
• Format: Article
• Language: English
• Subjects: Absorption; Adult; beta Carotene; Biological and medical sciences; Carbon Isotopes; Carotenoids - administration & dosage; Carotenoids - blood; Carotenoids - chemistry; Chromatography, High Pressure Liquid; Enzymes. Coenzymes. Vitamins. Pigments; Female; Fundamental and applied biological sciences. Psychology; Humans; Intestinal Mucosa - metabolism; Isomerism; Kinetics; Male; Metabolisms and neurohumoral controls; Vertebrates: anatomy and physiology, studies on body, several organs or systems; Vitamin A - blood
• ISSN: 0002-9165; 1938-3207
• DOI: 10.1093/ajcn/64.2.177

Absorption and metabolism of [13C]9-cis-beta-carotene ([13C]9c beta C) was studied in three subjects after a single oral dose. Subjects given 1.0 mg [13C]beta-carotene (mean: 99.4% 9-cis-beta-carotene, 0.6% all-trans-beta-carotene; dose A) had substantial concentrations of [13C]all-trans-beta-carotene ([13C]tr beta C) and [13C]all-trans retinol ([13C]retinol) but very low concentrations of [13C]cis-beta-carotene ([13C]cis beta C) 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]tr beta C and [13C]retinol derived from [13C]9c beta C, a second set of studies in the same subjects was performed with the same isomeric composition except with 13C labeling only in all-trans-beta-carotene (dose B). The results indicated that > 95% of plasma [13C]tr beta C and [13C]retinol observed after dose A was derived from [13C]9c beta C. The concentrations of [13C]tr beta C observed, in excess of that derived from the trace amounts of [13C]tr beta C in the dose, indicated that a significant proportion of the [13C]9c beta C dose was isomerized to [13C]tr beta C before entering the bloodstream. Although precise quantitative estimates of the extent of isomerization 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-carotene contributed to the near absence of postprandial plasma 9-cis-beta-carotene after its oral administration in humans. The observation of different ratios of beta-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-beta-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-beta-carotene.