Department of Pharmacology, University of Florida, Gainesville, Florida 32610.
Plant Physiol. 1988 Apr;86(4):1185-92. doi: 10.1104/pp.86.4.1185.
The role of the photosystems in the exchange of (18)O between species of inorganic carbon and water was studied in suspensions of the cyanobacterium Synechococcus sp. (UTEX 2380) using membrane-inlet mass spectrometry. This (18)O exchange is caused by the hydration-dehydration cycle of CO(2) and is catalyzed by carbonic anhydrase. We observed the complex (18)O exchange kinetics including dark-light-dark transients in suspensions of whole cells and found these to be identical to the (18)O exchange kinetics of physiologically fully active spheroplast preparations. There was no enhancement effect of inorganic nitrogen on inorganic carbon accumulation. Membrane preparations exhibited no uptake of inorganic carbon and very little carbonic anhydrase activity, although these membranes were photosynthetically fully competent. DCMU, the inhibitor of photosystem II, eliminated almost entirely the (18)O exchange activity of whole cells in the light. But this effect of DCMU could be reversed by addition of the electron donor couple 3,6-diaminodurene/ascorbate, suggesting the involvement of photosystem I in the events leading to (18)O exchange. Iodoacetamide, an inhibitor of CO(2) fixation, enhanced the (18)O exchange in whole cell suspensions and inhibited neither the uptake of inorganic carbon nor the dehydration of bicarbonate in the light. The proton carrier carbonylcyanide m-chlorophenylhydrazone and the inhibitors diethylstilbestrol and N,N' -dicyclohexyl carbodiimide affecting the membrane potential, totally abolished (18)O exchange in the light. From (18) O-labeled inorganic carbon experiments we conclude that one of the roles of photosystem I is to provide the active uptake of inorganic carbon into the cells, where carbonic anhydrase catalyzes the interconversion between CO(2) and HCO(3) (-) resulting in the (18)O exchange from inorganic carbon to water.
用膜进样质谱法研究了在蓝藻集胞藻(UTEX 2380)悬浮液中光系统在无机碳和水之间的(18)O 交换作用。这种(18)O 交换是由 CO2 的水合-脱水循环引起的,由碳酸酐酶催化。我们观察到整个细胞悬浮液中复杂的(18)O 交换动力学,包括黑暗-光-黑暗瞬变,发现这些与生理上完全活跃的原生质体制备物的(18)O 交换动力学相同。无机氮对无机碳积累没有增强作用。尽管这些膜具有完全的光合能力,但膜制剂对无机碳没有吸收作用,碳酸酐酶活性也非常低。作为光系统 II 抑制剂的 DCMU 几乎完全消除了整个细胞在光照下的(18)O 交换活性。但是,DCMU 的这种作用可以通过添加电子供体偶 3,6-二氨基二氢嘧啶/抗坏血酸来逆转,这表明光系统 I 参与了导致(18)O 交换的事件。作为 CO2 固定抑制剂的碘乙酰胺增强了整个细胞悬浮液中的(18)O 交换,并且在光照下既不抑制无机碳的吸收,也不抑制重碳酸盐的脱水。质子载体羰基氰化物 m-氯代苯腙和影响膜电位的抑制剂二乙基己基二硫代氨基甲酸钠和 N,N' -二环己基碳二亚胺完全消除了光照下的(18)O 交换。从(18)O 标记的无机碳实验中,我们得出结论,光系统 I 的作用之一是为无机碳的主动摄取提供细胞,在细胞中碳酸酐酶催化 CO2 和 HCO3-(-)之间的相互转化,导致(18)O 从无机碳交换到水中。