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Department of Biochemistry and Biophysics, Iowa State University, Ames, IA 50011
Two pathways have been suggested for the conversion of carotenoids to vitamin A in mammals, central cleavage and excentric cleavage. An enzyme, ß-carotenoid-15,15'-dioxygenase (EC 1.13.11.21), has been partly purified from the intestines of several species and has been identified in several other organs and species. The enzyme, which converts ß-carotene into two molecules of retinal in good yield, requires molecular oxygen and is inhibited by sulfhydryl-binding and iron-binding reagents. Most provitamin A carotenoids, including the ß-apo-carotenals, are cleaved to retinal by this enzyme. Its maximal activity in the rabbit is approximately 200 times that required to meet nutritional needs but is < 50% of that expected to produce signs of vitamin A toxicity. Excentric cleavage unquestionably occurs in plants and some microorganisms and might occur in mammals. Thus far, however, carotenoid dioxygenase with excentric bond specificity has been identified in mammals, the yield of ß-apo-carotenals from ß-carotene in vivo and in vitro is very low, and ß-apo-carotenals are formed nonbiologically from ß-carotene. To remain viable as an alternative pathway of vitamin A formation from carotenoids in mammals, the excentric cleavage hypothesis clearly requires unambiguous direct supporting evidence.
KEY WORDS: • carotenoids • ß-carotene • provitamin A • vitamin A • central cleavage • excentric cleavage • mammals • fish
1 Presented as part of the symposium, "Biological Actions of Carotenoids," given at the 72nd annual meeting of the Federation of American Societies for Experimental Biology, Las Vegas, NV, May 2, 1988, and supported by grants from the BASF Corporation, Hoffmann-LaRoche Inc., and the National Dairy Council.
2 Supported in part by research grants from the National Institutes of Health (DK 32793, DK 39733, CA 46406) and from the Competitive Grants Research Program, U.S. Department of Agriculture, No. 87-CRCR-1-2320.
3 Journal Paper No. J-13225 of the Iowa Agriculture and Home Economics Experiment Station, Ames IA. Project No. 2534.
Manuscript received 13 July 1988. Revision accepted 6 September 1988.
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