On the biosynthetic relationship between lycopene and colorless polyenes in tomatoes.

The suggestion of Zechmeister (1) that phytofluene might be a precursor of lycopene has been expanded by Porter and Lincoln (2), who proposed on the basis of experiments with tomatoes that a successive dehydrogenation of more saturated polyenes’ leads to the biosynthesis of lycopene and other carotenoids. This sequence would involve the intermediary formation of tetrahydrophytoene, phytoene, phytofluene, p-carotene, neurosporene, and lycopene. Although this hypothesis has received wide attention, objections based on genetic and other evidence have hecn raised t>o t,he validity of this concept (3). Goodwin has determined the concentration of some of these postulated intermediates at intervals during the ripening of tomatoes at various temperatures and has concluded that the Porter-Lincoln hypothesis could not account for the results obtained (4). An alternative scheme which was therefore proposed by Goodwin and Jamikorn (4) and Goodwin (5) and by MaeKinney (6) and Jenkins and MacKinney (7) suggests instead that the different polyenes might originate from one single large precursor by parallel pathways. A third possibility concerning the biosynthetic relationship between the various polyenes is one in which lycopene is the source of the more saturated polyenes and is converted to these compounds by successive reduction (4). Although changes in concentration of substances during the course of metabolic transformations often yield useful information concerning pathways, the interpretation of such data may occasionally be subject to error because an intermediate may be present in small quantity, may not change in concentration, and yet may have a high rate of turnover. This objection can often be overcome by the use of isotopes. In the study reported in this paper, 2-C14-acetate has been added to ripening tomatoes previously