Biosynthesis of the prenyl side chains of plastiquinone and related compounds in maize and barley shoots

Abstract

The incorporation of 14C by etiolated maize and barley shoots exposed to light of 14CO2 and [2-14C]mevalonic acid into phylloquinone, plastoquinone, ubiquinone, α-tocopherolquinone and α-tocopherol was examined. In maize (the principal tissue studied) it was demonstrated that 14C from [2-14C]mevalonic acid is incorporated into phylloquinone, plastoquinone and ubiquinone. α-Tocopherol and α-tocopherolquinone, although undoubtedly labelled from this substrate, were not purified completely. As expected, 14C from 14CO2 was incorporated into all components examined. Ozonolytic degradation studies showed that 14C from [2-14C]mevalonic acid was incorporated specifically into the prenyl side chains of plastoquinone and ubiquinone, and from this it was inferred that mevalonic acid can be regarded as the specific distal precursor to the prenyl portions of all terpenoid quinones occurring in plant tissues. From a comparison of the relative incorporation of 14C from 14CO2 and [2-14C]mevalonic acid into the intra- and extra-chloroplastidic terpenoids evidence was obtained consistent with the tenet that the prenyl portions of the chloroplastidic quinones phylloquinone and plastoquinone, along with β-carotene, are biosynthesized within the confines of the chloroplast, the side chain of the extraplastidic ubiquinone and phytosterols being synthesized elsewhere within the cell. The results obtained for the incorporation of 14C from 14CO2 and [2-14C]mevalonic acid into α-tocopherol and α-tocopherolquinone were not readily interpretable with regard to the site of synthesis of these compounds.