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

Absorption and metabolism of (13C)9-cis-f3-car­ otene ((13C)9cf3C) was studied in three subjects after a single oral dose. Subjects given 1.0 mg e 3 C)f3-carotene (mean: 99.4% 9-cis­ f3-carotene, 0.6% all~trans-f3-car otene; dose A) had substantial concentrations of (13C)all-trans-f3-carotene ((13C)trf3C) and (13C)all-trans retinol ((13C)retinol) but very low concentrations of ( 13 C)cis-f3-carotene ((13C)cisf3c) in saponified plasma 5 h after dosing, as determined by HPLC and isotope-ratio mass spectrom­ etry. There was no evidence of appreciable absorption of (13C)9- cis retinol. To determine the proportion of (13C)trf3C and (13C)re­ tinol derived from (13C)9cf3C, a second set of studies in the same subjects was performed with the same isomeric composition ex­ cept with 13C labeling only in all-trans-f3-carotene (dose B). The results indicated that> 95% of plasma (13C)trf3C and (13C)retinol observed after dose A was derived from (13C)9cf3C. The concen­ trations of (13C)trf3C observed, in excess of that derived from the trace amounts of (13C)trf3C in the dose, indicated that a significant proportion of the (13C)9cf3C dose was isomerized to (13C)trf3C before entering the bloodstream. Although precise quantitative estimates of the extent of isomerization of 9-cis-f3-carotene could not be made, it is apparent that cis-trans isomerization of 9-cis-f3- carotene to all-trans-f3-carotene contributed to the near absence of postprandial plasma 9-cis-f3-carotene after its oral administration in humans. The observation of different ratios of f3-carotene to retinol between the two dosing protocols suggests that isomeriza­ tion did not occur exclusively before uptake by the intestinal mucosa. These results indicate that isomerization of ingested 9-cis­ f3-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-f3-carotene. Am J Clin Nutr has been estimated to be as high as 57% that of all-trans-f3- carotene, indicating substantial conversion of the 9-cis stereo­ isomer (7). Several studies have reported serum concentrations of 9-cis-f3-carotene after ingestion of mixtures of all-trans- and 9-cis-f3-carotene in humans and animals. Human trials have consistently observed that plasma concentrations of all-trans­ f3-carotene could be elevated in response to dietary intake, whereas those of the 9-cis isomer were not increased even after high amounts of this isomer were fed (8, 9). Recently, similar findings were reported after chylomicron carotene analysis (10), suggesting that isomeric enrichment in favor of the all­ trans isomer took place in the intestinal mucosa. A similar finding was reported in ferrets (11). Rats and chickens fed mixtures of these f3-carotene isomers accumulated both iso­ mers in liver tissue (12-14), suggesting that 9-cis-f3-carotene may be absorbed but rapidly deposited in tissues, thus resulting in low blood concentrations of this isomer. Several possible mechanisms may explain the near. absence of 9-cis-{3-carotene in human plasma, including 1) inefficient intestinal uptake of the 9-cis isomer relative to the all-trans isomer, 2) extensive metabolism of 9-cis-{3-carotene to vitamin A in the intestinal mucosa, 3) extensive isomerization of 9-cis-{3-carotene to all­ trans-{3-carotene or other cis isomers subsequent to ingestion, 4) rapid elimination of 9-cis-{3-carotene from plasma, or 5) extensive degradation of 9-cis-{3-carotene in the gastrointesti­ nal tract. To investigate many of these mechanisms, a means of track­ ing ingested 9-cis-{3-carotene molecules is required, ie, a tracer approach. The current study used (13C)9-cis-{3-carotene ((13C)9c{3C) to gain insight regarding the fate of 9-cis-{3- carotene in adult humans. The study of metabolism of com­ pletely 13C substituted (13C)all-trans-{3-carotene ((13C)tr{3C) using gas chromatography-combustion-isotope-ratio mass spectrometry (GCC-IRMS) was described recently (15), and here we extend this approach to the study of (13C)9c{3C. The use of high-precision gas isotope-ratio mass spectrometry in biomedical research was reviewed recently (16).