| Budget Amount *help |
¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 1993: ¥300,000 (Direct Cost: ¥300,000)
Fiscal Year 1992: ¥1,600,000 (Direct Cost: ¥1,600,000)
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| Research Abstract |
Natural sweet compound accumulated in Stevia rebaudiana Bertoni are glucosides whose aglycone is steviol. Kaurenoic acid is the precousor of the plant growth regulator, gibellelic acid, and as well as of steviol. Kaurenoic acid and many functional compounds such as sterols, carotenoids, quinones and phytol-moiety of chlorophylls are synthesized via mevalonic pathway whose rate-limiting enzyme is hydroxy methyl-glutaryl-CoA (HMG-CoA) reductase.
To evaluate the possiblity that S.rebaudiana synthesize abonormal amonunt of mevalonic acid, and metabolize it to non-functinal stevol via kaurenoic acid avoiding the accumulation of giberellic acid, we compared HMG-CoA reductase activities in isolated chloroplasts, microsomal and soluble fraction from S.rebaudiana, Solidago altissima L.and Spinach leaves. Although specific activities of HMG-CoA reductase detected in microsomal and soluble fractions from three different kinds of leaves were almost the same level, the activity in chloroplasts were 200-300-fold and 8-10-fold higher than that from spinach and that from S.altissima. Km values for the substrate were the same order and the inhibition by mevalonin, a specific inhibitor for HMG-CoA reductase caused similar inhibition. These results indicate that S.rebaudiana synthesizes higher amount of mevalonic acid which is endogenous stress, and metabolizes it to kaurenoic acid and then to steviol avoiding accumulation of more gibellelic acid. As steviol is hydrophobic nature, glucosylation to its skeletone endowes water-solubility making it possible to across biomembrenes. Thus the accumulation of sweet stevioside compounds in S.rebaudia is concluded to be a response to overcome the highly accumulated mevalonic acid.
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