Biosynthesis of phytoquinones. Incorporation of l-[Me−¹⁴C³H]methionine into terpenoid quinones and chromanols in maize shoots

1. Radioactivity from l-[Me−¹⁴C,³H]methionine is incorporated into phylloquinone, plastoquinone, γ-tocopherol, α-tocopherol, α-tocopherolquinone and ubiquinone in maize shoots. 2. Comparative studies with other terpenoids (squalene and β-carotene) and chemical degradation of selected quinones (ubiquinone and plastoquinone) established that all the radioactivity is confined to nuclear methyl substituents. 3. In ubiquinone 76% of the radioactivity is in the methoxyl groups and 24% in the ring C-methyl group. 4. Taking the phytosterols as an internal reference and accepting the atomic ratio of ¹⁴C/³H transferred from l-[Me−¹⁴C,³H]methionine to the supernumerary group at C₂₄ to be 1:2 the ratio of all the quinones and chromanols examined approached 1:3. After allowing for the fact that for plastoquinone, γ-tocopherol, α-tocopherol and α-tocopherolquinone one nuclear methyl group is formed from the β-carbon of tyrosine, these results show that one nuclear C-methyl group for phylloquinone, plastoquinone and γ-tocopherol, two nuclear methyl groups for α-tocopherol and α-tocopherolquinone and one nuclear methyl and two methoxyl groups for ubiquinone are formed by the transfer of intact methyl groups from methionine. 5. From a comparison of the incorporation of ¹⁴C radioactivity into these compounds it would appear that the methylation reactions involved in phylloquinone and plastoquinone biosynthesis take place in the chloroplast, whereas those involved with ubiquinone biosynthesis occur else-where within the cell.