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Mechanism of the Biosynthesis of Vitamin A from ß-Carotene

From the ¹ From the Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York 10032

Experiments were conducted to determine whether the hydrogen atoms attached to the 2 central carbon atoms of ß-carotene are retained or are lost during the conversion of ß-carotene to vitamin A. Doubly labeled ß-carotene, uniformly labeled with ¹⁴C throughout the molecule but specifically labeled with ³H only at the central carbon atoms (C-15 and C-15'), was fed to each of two lymph fistula rats. Lymph was collected and the retinyl esters isolated. The ratio of ³H to ¹⁴C in the retinyl esters from lymph, and in the retinol obtained from these esters by saponification, was identical with the ratio of ³H to ¹⁴C in the dietary ß-carotene. This indicated that there was no loss of ³H relative to ¹⁴C during the conversion of ß-carotene to vitamin A. Complete retention of ³H relative to ¹⁴C was also found during the conversion in vitro of doubly labeled ß-carotene to retinal with homogenates of rat intestinal mucosa. The hydrogen atoms attached to the central carbon atoms of ß-carotene must therefore be retained completely during the biosynthesis of vitamin A.

In order to verify that the ³H label was attached only to the central carbon atoms of ß-carotene, the retinol (from lymph) was oxidized to retinoic acid by a two-step procedure. This oxidation was accompanied by a complete loss of ³H label from the molecule.

The biosynthesis of vitamin A from ß-carotene is most likely a dioxygenase reaction, in which molecular oxygen reacts with the 2 central carbon atoms of ß-carotene, followed by the cleavage of the central double bond of ß-carotene to yield 2 molecules of retinal.

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