PRODUCTION OF USEFUL COMPOUND COUPLED TO RESPONSE OF ENDOGENOUS STRESS

甜叶菊中积累的天然甜味化合物是糖苷类化合物，其苷元为甜菊醇。高岭酸是植物生长调节剂赤霉酸的前体，也是甜菊醇的前体。以羟甲基戊二酰辅酶A(HMG-CoA)还原酶为限速酶，通过甲酮酸途径合成了高山酸和多种功能化合物，如甾醇、类胡萝卜素、苯二酚和植物醇类等。为了评价白花蛇舌草合成甲戊二酰辅酶A(HMG-CoA)的能力，并通过龙葵酸将其代谢成无功能甜菊，避免赤霉酸的积累，我们比较了白花蛇舌草、一枝黄花和菠菜叶片的叶绿体、微体和可溶性部分的HMG-CoA还原酶活性。3种不同叶片的微粒体和可溶性部分的HMG-CoA还原酶比活力几乎相同，但叶绿体中的HMG-CoA还原酶比活力分别是菠菜和高山松的200-300倍和8-10倍。底物的Km值为同一量级，HMG-CoA还原酶的特异性抑制剂甲伐他宁的抑制作用与此相似。这些结果表明，雷公藤合成了较高数量的内源胁迫甲氧戊酸，并将其代谢为棕榈酸，然后代谢为甜菊醇，避免了更多的赤霉酸的积累。由于甜菊醇是疏水性的，其骨架的糖基化赋予了其水溶性，使其有可能跨越生物膜。因此，甜叶菊中甜菊糖苷化合物的积累是对高度积累的甲伐他酸的反应。

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.