Incorporation of (14)CO 2 in prenylquinones of Chlorella pyrenoidosa.

Incorporation and release of (14)C-label in prenylquinones of Chlorella was investigated under steady state conditions. After one hour of (14)CO2-photosynthesis all plastid quinones investigated were labeled. The highest label was found in phylloquinone (18%) while α-tocopherol exhibits the lowest label (0.38%). Among the plastoquinones, plastohydroquinone-9 shows a higher labeling degree (5.1%) and a faster labeling kinetic than plastoquinone-9 (1.6%). After replacement of (14)CO2 against (12)CO2 the total radioactivity in plastohydroquinone-9, α-tocopherol and phylloquinone decreases but in α-tocoquinone and plastoquinone-9 proceeds further. From this labeling kinetic we conclude, that newly synthesized [(14)C]α-tocopherol molecules are converted to [(14)C]α-tocoquinone and [(14)C]plastohydroquinone-9 molecules to [(14)C]plastoquinone-9. From their (14)C-incorporation kinetic half-lives could be calculated for all prenylquinones in the same ranges as previously found for the chlorophylls and carotenoids (Grumbach et al., 1978). Half-lives are shorter in plastohydroquinone-9 (30 min) and plastoquinone-9 (40 min) than in phylloquinone (55 min), α-tocoquinone (50 min) and α-tocopherol (220 min). This means that all prenyl-lipids such as chlorophyll a, α-and β-carotene, plastohydroquinone-9 and plastoquinone-9 which are more directly involved in the process of photosynthesis are subject to a continuous and higher turnover than the xanthophyll and α-tocopherol. From the fast labeling kinetic and short half-lives of α-tocoquinone and especially phylloquinone with a labeling degree of 12% after one hour of (14)CO2 photosynthesis we suppose that perhaps these two prenylquinones are also involved in the photosynthetic activity of chloroplasts.