Kocsis M G, Hanson A D
Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611, USA.
Plant Physiol. 2000 Jul;123(3):1153-61. doi: 10.1104/pp.123.3.1153.
3-Dimethylsulfoniopropionate (DMSP) is an osmoprotectant accumulated by the cordgrass Spartina alterniflora and other salt-tolerant plants. Previous in vivo isotope tracer and metabolic modeling studies demonstrated that S. alterniflora synthesizes DMSP via the route S-methyl-Met --> 3-dimethylsulfoniopropylamine (DMSP-amine) --> 3-dimethylsulfoniopropionaldehyde --> DMSP and indicated that the first reaction requires a far higher substrate concentration than the second to attain one-half-maximal rate. As neither of these reactions is known from other organisms, two novel enzymes are predicted. Two corresponding activities were identified in S. alterniflora leaf extracts using specific radioassays. The first, S-methyl-Met decarboxylase (SDC), strongly prefers the L-enantiomer of S-methyl-Met, is pyridoxal 5'-phosphate-dependent, generates equimolar amounts of CO(2) and DMSP-amine, and has a high apparent K(m) (approximately 18 mM) for its substrate. The second enzyme, DMSP-amine oxidase (DOX), requires O(2) for activity, shows an apparent K(m) for DMSP-amine of 1.8 mM, and is not accompanied by DMSP-amine dehydrogenase or transaminase activity. Very little SDC or DOX activity was found in grasses lacking DMSP. These data indicate that SDC and DOX are the predicted novel enzymes of DMSP synthesis.
3-二甲基巯基丙酸内盐（DMSP）是一种渗透保护剂，由互花米草和其他耐盐植物积累。先前的体内同位素示踪和代谢建模研究表明，互花米草通过S-甲基蛋氨酸→3-二甲基巯基丙胺（DMSP-胺）→3-二甲基巯基丙醛→DMSP的途径合成DMSP，并表明第一个反应达到最大反应速率一半时所需的底物浓度远高于第二个反应。由于其他生物体中均未发现这两种反应，因此预测存在两种新酶。使用特定的放射性测定法在互花米草叶片提取物中鉴定出了两种相应的活性。第一种是S-甲基蛋氨酸脱羧酶（SDC），它强烈偏好S-甲基蛋氨酸的L-对映体，依赖于磷酸吡哆醛5'-磷酸，产生等摩尔量的CO₂和DMSP-胺，并且对其底物具有较高的表观Kₘ（约18 mM）。第二种酶是DMSP-胺氧化酶（DOX），其活性需要O₂，对DMSP-胺的表观Kₘ为1.8 mM，并且不伴有DMSP-胺脱氢酶或转氨酶活性。在缺乏DMSP的草中几乎未发现SDC或DOX活性。这些数据表明，SDC和DOX是预测的DMSP合成新酶。
