Biosynthetis of phytoquinones. Biosynthetic origins of the nuclei and satellite methyl groups of plastoquinone, tocopherols and tocopherolquinones in maize shoots, bean shoots and ivy leaves

Abstract

1. By using dl-[ring−14C]phenylalanine, dl-[β−14C]phenylalanine, dl-[α−14C]-tyrosine and dl-[β−14C]tyrosine it was shown that in maize shoots (Zea mays) the nucleus and one nuclear methyl group of each of the following compounds, plastoquinone, γ-tocopherol (aromatic nucleus) and α-tocopherolquinone, are formed from the nuclear carbon atoms and β-carbon atom respectively of either exogenous phenylalanine or exogenous tyrosine. With ubiquinone only the aromatic ring of the amino acid is used in the synthesis of the quinone nucleus. Chemical degradation of plastoquinone and γ-tocopherol molecules labelled from l-[U−14C]tyrosine established that a C6–C1 unit directly derived from the amino acid is involved in the synthesis of these compounds. Radioactivity from [β−14C]cinnamic acid is not incorporated into plastoquinone, tocopherols or tocopherolquinones, demonstrating that the C6–C1 unit is not formed from any of the C6–C1 phenolic acids associated with the metabolism of this compound. 2. The incorporation of radioactivity from l-[U−14C]tyrosine, dl-[β−14C]tyrosine and dl-[U−14C]phenylalanine into bean shoots (Phaseolus vulgaris) and dl-[β−14C]tyrosine and l-[Me−14C]methionine into ivy leaves (Hedera helix) was also investigated. Similar results were obtained to those reported for maize, except that in beans phenylalanine is only used for ubiquinone biosynthesis. This is attributed to the absence of phenylalanine hydroxylase from these tissues. In ivy leaves it is found that the β-carbon atom of tyrosine gives rise to the 8-methyl group of δ-tocopherol, and it is suggested that for all other compounds examined it will give rise to the nuclear methyl group meta to the polyprenyl unit. 3. Preliminary investigations with the alga Euglena gracilis showed that in this organism ring-opening of tyrosine occurs to such an extent that the incorporation data from radiochemical experiments are meaningless. 4. The above results, coupled with previous observations, are interpreted as showing that in higher plants the nucleus of ubiquinone can be formed from either phenylalanine or tyrosine by a pathway involving as intermediates p-coumaric acid and p-hydroxybenzoic acid. Plastoquinone, tocopherols and α-tocopherolquinone are formed from p-hydroxyphenylpyruvate by a pathway in which the aromatic ring and C-3 of the side chain give rise respectively to the nucleus and to one nuclear methyl group. 5. Dilution experiments provided evidence that in maize shoots p-hydroxyphenylpyruvic acid and homogentisic acid (produced from p-hydroxyphenylpyruvic acid) are involved in plastoquinone biosynthesis, and presumably the biosynthesis of related compounds: however, other possible intermediates in the conversion including toluquinol (the aglycone of the proposed key intermediate) showed no dilution effects. Further, radioactivity from [Me−14C]toluquinol is not incorporated into any of the compounds examined. 6. Dilution experiments with 3,4-dihydroxybenzaldehyde and radioactive-labelling experiments with 3,4-dihydroxy[U−14C]benzoic acid demonstrated that these compounds are not involved in the biosynthesis of either ubiquinone or phylloquinone in maize shoots. 7. Evidence is also presented to show that in maize shoots ring-opening of the aromatic amino acids takes place. The suggestion is offered that this may take place via homogentisic acid, as in animals and some micro-organisms.