Evidence of cis-trans isomerization of 9-cis-beta-carotene during absorption in humans.
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.
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Division of Nutritional Sciences, Cornell University, Ithaca, NY 14853, USA.
Parker
R S
RS
Goodman
K J
KJ
Swanson
J E
JE
Corso
T N
TN
D000042:Absorption; D000328:Adult; D002247:Carbon Isotopes; D002338:Carotenoids / Q000008:administration & dosage / Q000097:blood* / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D005260:Female; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism; D007536:Isomerism; D007700:Kinetics; D008297:Male; D014801:Vitamin A / Q000097:blood; D019207:beta Carotene
D002247:Carbon Isotopes; D019207:beta Carotene; D014801:Vitamin A; D002338:Carotenoids