A Carotene Precursor: Its Proposed Structure and Place in Biosynthetic Sequence

The isoprenoids, cholesterol and rubber, are known to arise from mevalonic (3,5-dihydroxy-3-methylvaleric) acid (5, 7). The same is true of carotenes (1, 6). Thus it has been found in this laboratory that mevalonic acid is rapidly incorporated into carotenes and related compounds in the tomato fruit (6). The hydrocarbon fraction of tomatoes previously incubated with 2-C14-mevalonate contains radioactivity, however, not only in the known carotenes but also in a previously undescribed, · colorless polyene which possesses absorption maxima at 208 nw and 231 mμ, The high specific activity of this compound, which has been referred to as fraction II (6), suggests that it may be a precursor of carotenes. This paper reports further experiments pertaining to the identity of fraction II and to its role in the biosynthesis of carotenoids.