I. The biosynthesis of open chain terpenes in plants. II. Fractionation of the stable carbon isotopes in plants

摘要

I: Open chain terpene synthesis in plants was studied by measurement of the incorporation of potential intermediates into the rubber of the rubber producing plants Taraxacum kok saghyz and Hevea brasiliensis. Intact plants incorporate 1-C14-acetate and 2-C14-acetate into rubber without randomization of the label. [Beta]-Methylcrotonic acid was found to be an ineffective rubber precursor in intact plants.Enzymatic experiments were performed using Hevea latex as a source of enzyme. C14-acetate is rapidly incorporated into a volatile, non-acidic, non-polar substance in this system under anaerobic conditions. C14-acetate is not incorporated into rubber. Mevalonic acid is rapidly incorporated into rubber in this system. Partial degradation of the rubber indicates that no randomization occurs during incorporation. This result suggests that mevalonic acid is on the pathway of terpene synthesis in plants.II: The two stable carbon isotopes, C12 and C13, occur in nature in the ratio of about ninety to one. Various workers have shown that this ratio is not fixed, but may vary by as much as 5%. Interestingly enough, this variation is not random. Carbon reservoirs such as limestone, atmospheric CO2, land plants, algae and coal all exhibit characteristic C13/C12 ratios. This section of the dissertation is concerned with the differences between the C13/C12 ratios of plants and those of the carbon sources from which such plants have grown.Both algae and terrestrial plants have smaller C13/C12 ratios than those of dissolved carbonates and atmospheric CO2 respectively. The magnitude of this fractionation was determined for tomato plants by growing the plants from seed in CO2 of known isotopic composition. Separation of the plant material into its component chemical constituents showed that only the lipid fraction differed markedly in C13/C12 ratio from that of the plant as a whole. The lipid fraction is enriched in C12 and possesses a C13/C12 ratio similar to that of petroleums derived from land plants. A similar relation was found to exist between marine algae, their lipids, and petroleums of marine origin. The CO2 evolved by plant respiration is slightly enriched in C13 as compared to the plant. This process apparently closely related to the C12 enrichment in lipid fractions.A possible mechanism for fractionation of C13 and C12 in photosynthesis is suggested. This suggestion is supported by observations of the C13/C12 ratio of CO2 dissolved in higher plants and by determination of the fractionation which occurs during fixation of CO2 by the photosynthetic carboxylation enzyme.