Mechanism of the Biosynthesis of Vitamin A from β-Carotene

Abstract

Experiments were conducted to determine whether the hydrogen atoms attached to the 2 central carbon atoms of β-carotene are retained or are lost during the conversion of β-carotene to vitamin A. Doubly labeled β-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 β-carotene. This indicated that there was no loss of 3H relative to 14C during the conversion of β-carotene to vitamin A. Complete retention of 3H relative to 14C was also found during the conversion in vitro of doubly labeled β-carotene to retinal with homogenates of rat intestinal mucosa. The hydrogen atoms attached to the central carbon atoms of β-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 β-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 β-carotene is most likely a dioxygenase reaction, in which molecular oxygen reacts with the 2 central carbon atoms of β-carotene, followed by the cleavage of the central double bond of β-carotene to yield 2 molecules of retinal.