Biosynthesis of 9-cis-Retinoic Acid from 9-cis-β-Carotene in Human Intestinal Mucosa in Vitro

doi:10.1006/abbi.1994.1371

Archives of Biochemistry and Biophysics

Volume 313, Issue 1, August 1994, Pages 150-155

https://doi.org/10.1006/abbi.1994.1371 Get rights and content

Abstract

The role of 9-cis-β-carotene (9-cis-β-C) as a potential precursor of 9-cis-retinoic acid (9-cis-RA) has been examined in human intestinal microcosa in vitro. By using HPLC, uv spectra, and chemical derivatization analysis, both 9-cis-RA and all-trans-retinoic acid (all-trans-RA) have been identified in the postnuclear fraction of human intestinal microcosa after incubation with 9-cis-β-C at 37°C. The biosynthesis of both 9-cis-RA and all-trans-RA from 9-cis-β-C was linear with increasing concentrations of 9-cis-β-C (2-30 μM) and was linear with respect to tissue protein concentration up to 0.75 mg/ml. Retinoic acid was not detected when a boiled incubation mixture was incubated in the presence of 9-cis-β-C. The rate of synthesis of 9-cis- and all-trans-RA from 4 μM 9-cis-β-C were 16 ± 1 and 18 ± 2 pmol/hr/mg of protein, respectively. However, when 2 μM all-trans-β-C was added to the 4 μM 9-cis-β-C, the rate of all-trans-RA synthesis was increased to 38 ± 6 pmol/hr/mg of protein, whereas the rate of 9-cis-RA synthesis remained the same. These results suggest that 9-cis-RA is produced directly from 9-cis-β-C. Furthermore, incubations of either 0.1 μM 9-cis- or all-trans-retinal under the same incubation conditions showed that 9-cis-RA could also arise through oxidative conversion of 9-cis-retinal. Although only 9-cis-RA was detected when 9-cis-RA was used as the substrate, the isomerization of the all-trans-RA to 9-cis-RA cannot be ruled out, since both all-trans-RA and trace amounts of 9-cis-RA were detected when all-trans-retinal was incubated as the substrate. These data indicate that 9-cis-β-C can be a source of 9-cis-RA in the human. This conversion may have a significance in the anticarcinogenic action of β-C.

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Cited by (89)

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Citation Excerpt :
Notably, Jim Olson's group previously obtained a similar result with cell free extracts of rat intestine [192], indicating that BCO1 possesses intrinsic isomerase activity. This finding contrasts with reports suggesting that 9-cis-β-carotene is converted to 9-cis-retinal and all-trans-retinal in one to one molar ratio [193,194]. It also contrasts the more recent proposal that 9-cis-β-carotene is not enzymatically converted by recombinant human BCO1 [88].
Carotenoids exert a rich variety of physiological functions in mammals and are beneficial for human health. These lipids are acquired from the diet and metabolized to apocarotenoids, including retinoids (vitamin A and its metabolites). The small intestine is a major site for their absorption and bioconversion. From here, carotenoids and their metabolites are distributed within the body in triacylglycerol-rich lipoproteins to support retinoid signaling in peripheral tissues and photoreceptor function in the eyes. In recent years, much progress has been made in identifying carotenoid metabolizing enzymes, transporters, and binding proteins. A diet-responsive regulatory network controls the activity of these components and adapts carotenoid absorption and bioconversion to the bodily requirements of these lipids. Genetic variability in the genes encoding these components alters carotenoid homeostasis and is associated with pathologies. We here summarize the advanced state of knowledge about intestinal carotenoid metabolism and its impact on carotenoid and retinoid homeostasis of other organ systems, including the eyes, liver, and immune system. The implication of the findings for science-based intake recommendations for these essential dietary lipids is discussed.
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Citation Excerpt :
The major central cleavage pathway, via β-carotene 15,15′-monooxygenase or BCO1, generates retinoids from provitamin A carotenoids [110–113], such as α- and β-carotene and β-cryptoxanthin. An alternative eccentric cleavage pathway is catalyzed by β-carotene 9′,10′-dioxygenase or BCO2 [114–116]. BCO2 is expressed in several tissues not known to be sensitive to vitamin A deficiency and/or that lack BCO1 expression [117].
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Citation Excerpt :
These isomers have also been detected in human plasma (19, 20). Cleavage activity with these β-carotene isomers have been shown by incubation with intestine and liver homogenates (14, 21, 22). However, the use of cell homogenates to study enzymatic oxidation of carotenoids in vitro is problematic.
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Citation Excerpt :
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Regular ArticleBiosynthesis of 9-cis-Retinoic Acid from 9-cis-β-Carotene in Human Intestinal Mucosa in Vitro

Abstract

Regular Article
Biosynthesis of 9-cis-Retinoic Acid from 9-cis-β-Carotene in Human Intestinal Mucosa in Vitro