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-C-13,3-H-2]-deox yxylulose or [2-C-13,4-H-2]1-deoxyxylulose. Lutein and chlorophylls were is olated from the cell mass, and hydrolysis of the chlorophyll mixtures affor ded phytol. Isotope labeling patterns of phytol and lutein were determined by H-2 NMR and H-1,H-2-decoupled C-13 NMR. From the data it must be conclud ed that the deuterium atom in position 3 of deoxyxylulose was incorporated into both isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate w ith a rate of 75% (with respect to the internal C-13 label). The detected s tereochemical signature implies that the label is located preferentially in the (E)-hydrogen atom of IPP. This preferential labeling, in turn, rules o ut dimethylallyl pyrophosphate as the compulsory precursor of IPP. In the e xperiment with [2-C-13,4-H-2]1-deoxyxylulose, the C-13 label was efficientl y transferred to the terpenoids whereas the H-2 label was completely washed out, most probably after IPP formation as a consequence of the isomerizati on and elongation process. In addition, the data cast light on the stereoch emical course of the dehydrogenation and cyclization steps involved in the biosynthesis of lutein.