Cleavage to Vitamin A

Provitamin A carotenoids are sources of vitamin A. Of the 50-60 carotenoids having provitamin A activity, ^-carotene, /3-cryptoxanthin, and a-carotene are the major sources of vitamin A nutrition in humans, ^-carotene being the most important (Figure 2). Vitamin A (retinol) and its derivatives retinal and retinoic acid perform vital functions in the vertebrate body. Retinal (11-ci's retinal) combined with opsin functions in the visual system in signal transduc-tion of light reception. Retinol and retinoic acid function in reproduction (spermatogenesis), growth regulation (general development and limb morphogenesis), and cell differentiation. Provitamin A activity requires at least one unsubstituted ^-ionone ring, the correct number and orientation of methyl groups along the polyene backbone, and the correct number of conjugated double bonds, preferably in the trans-isomer orientation. Two pathways for the formation of retinal from ^-carotene have been proposed. First, central cleavage by which ^-carotene 15,15'-mono- or dioxy-genase catalyzes ^-carotene cleavage to form two molecules of retinal, which can then be converted to retinol or retinoic acid (Figure 2). Some debate on the mechanism of the ^-carotene central cleavage enzyme still exists, but evidence leans towards activity as a monooxygenase, not a dioxygenase. Alternatively, in the eccentric cleavage pathway ^-carotene can be cleaved at any of the double bonds along the polyene backbone (other than the 15-15'double bond). Products of these reactions (apocarotenals) are then further metabolized to retinoic acid and retinol. An asymmetric cleavage enzyme has recently been cloned that cleaves ^-carotene at the 9'-10'-double bond to form ^-ionone and /3-apo-10'-carotenal. The discovery of this enzyme indicates at least some eccentric cleavage occurs in vertebrates. This eccentric cleavage process has been proposed to occur during more oxidative conditions, while central cleavage would predominate under normal physiological conditions. Central cleavage is considered to be the major pathway because of the scarcity of eccentric cleavage products detected in vivo.

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