Guy R. D., Fogel M. L., Berry J. A.
Department of Plant Biology, Carnegie Institution of Washington, Stanford, California 94305-1297 (R.D.G., J.A.B.).
Plant Physiol. 1993 Jan;101(1):37-47. doi: 10.1104/pp.101.1.37.
Isotope discrimination during photosynthetic exchange of O2 and CO2 was measured using enzyme, thylakoid, and whole cell preparations. Evolved oxygen from isolated spinach thylakoids was isotopically identical (within analytical error) to its source water. Similar results were obtained with Anacystis nidulans Richter and Phaeodactylum tricornutum Bohlin cultures purged with helium. For consumptive reactions, discrimination ([delta], where 1 + [delta]/1000 equals the isotope effect, k16/k18 or k12/k13) was determined by analysis of residual substrate (O2 or CO2). The [delta] for the Mehler reaction, mediated by ferredoxin or methylviologen, was 15.3[per mille (thousand) sign]. Oxygen isotope discrimination during oxygenation of ribulose-1,5-bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (Rubisco) was 21.3[per mille (thousand) sign] and independent of enzyme source, unlike carbon isotope discrimination: 30.3[per mille (thousand) sign] for spinach enzyme and 19.6 to 23[per mille (thousand) sign] for Rhodospirillum rubrum and A. nidulans enzymes, depending on reaction conditions. The [delta] for O2 consumption catalyzed by glycolate oxidase was 22.7[per mille (thousand) sign]. The expected overall [delta] for photorespiration is about 21.7[per mille (thousand) sign]. Consistent with this, when Asparagus sprengeri Regel mesophyll cells approached the compensation point within a sealed vessel, the [delta]18O of dissolved O2 came to a steady-state value of about 21.5[per mille (thousand) sign] relative to the source water. The results provide improved estimates of discrimination factors in several reactions prominent in the global O cycle and indicate that photorespiration plays a significant part in determining the isotopic composition of atmospheric oxygen.
利用酶、类囊体和全细胞制剂测量了光合作用中氧气和二氧化碳交换过程中的同位素分馏。分离的菠菜类囊体释放出的氧气与其源水在同位素上相同（在分析误差范围内）。用氦气吹扫的蓝纤维藻和三角褐指藻培养物也得到了类似结果。对于消耗性反应，通过分析残余底物（氧气或二氧化碳）来确定分馏值（δ，其中1 + δ/1000等于同位素效应，k16/k18或k12/k13）。由铁氧还蛋白或甲基紫精介导的梅勒反应的δ值为15.3‰。与碳同位素分馏不同，由核酮糖-1,5-二磷酸羧化酶/加氧酶（Rubisco）催化的核酮糖-1,5-二磷酸（RuBP）加氧过程中的氧同位素分馏为21.3‰，且与酶源无关：菠菜酶的碳同位素分馏为30.3‰，红螺菌和蓝纤维藻酶的碳同位素分馏为19.6至23‰，具体取决于反应条件。乙醇酸氧化酶催化的氧气消耗的δ值为22.7‰。光呼吸的预期总δ值约为21.7‰。与此一致的是，当石刁柏叶肉细胞在密封容器中接近补偿点时，溶解氧的δ18O相对于源水达到约21.5‰的稳态值。这些结果改进了对全球氧循环中几个重要反应的分馏因子的估计，并表明光呼吸在决定大气氧气的同位素组成方面起着重要作用。
