The stereospecific biosynthesis of phytoene and polyunsaturated carotenes.

1. The incorporation of [2-(14)C,(5R)-5-(3)H(1)]mevalonic acid and [2-(14)C,5-(3)H(2)]-mevalonic acid into phytoene, phytofluene, zeta-carotene, neurosporene, alpha-, beta-, gamma- and delta-carotene and lycopene by slices of fruit from two tomato mutants (delta and tangerine) and into alpha- and beta-carotene by bean leaves has been studied. 2. In the formation of phytoene, all the pro-R-hydrogen atoms from C-5 of mevalonic acid are retained whereas two pro-S-hydrogen atoms are lost. 3. Possible mechanisms for the condensation of two molecules of all-trans-geranylgeranyl pyrophosphate are outlined. 4. In each dehydrogenation step from phytoene to the fully unsaturated carotenes, one pro-R-hydrogen atom from C-5 of mevalonic acid is lost, indicating that the sequential dehydrogenation is stereospecific and in the same sense at each step.