Dimethylallyl Pyrophosphate Is Not the Committed Precursor of Isopentenyl Pyrophosphate during Terpenoid Biosynthesis from 1-Deoxyxylulose in Higher Plants

Cell cultures of Catharanthus roseus were supplied with [2-¹³C,3-²H]-deoxyxylulose or [2-¹³C,4-²H]1-deoxyxylulose. Lutein and chlorophylls were isolated from the cell mass, and hydrolysis of the chlorophyll mixtures afforded phytol. Isotope labeling patterns of phytol and lutein were determined by ²H NMR and ¹H,²H-decoupled ¹³C NMR. From the data it must be concluded that the deuterium atom in position 3 of deoxyxylulose was incorporated into both isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate with a rate of 75% (with respect to the internal ¹³C label). The detected stereochemical signature implies that the label is located preferentially in the (E)-hydrogen atom of IPP. This preferential labeling, in turn, rules out dimethylallyl pyrophosphate as the compulsory precursor of IPP. In the experiment with [2-¹³C,4-²H]1-deoxyxylulose, the ¹³C label was efficiently transferred to the terpenoids whereas the ²H label was completely washed out, most probably after IPP formation as a consequence of the isomerization and elongation process. In addition, the data cast light on the stereochemical course of the dehydrogenation and cyclization steps involved in the biosynthesis of lutein.