Espie G. S., Kandasamy R. A.
Department of Botany, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6.
Plant Physiol. 1994 Apr;104(4):1419-1428. doi: 10.1104/pp.104.4.1419.
The effect of monensin, an ionophore that mediates Na+/H+ exchange, on the activity of the inorganic carbon transport systems of the cyanobacterium Synechococcus UTEX 625 was investigated using transport assays based on the measurement of chlorophyll a fluorescence emission or 14C uptake. In Synechococcus cells grown in standing culture at about 20 [mu]M CO2 + HCO3-, 50 [mu]M monensin transiently inhibited active CO2 and Na+-independent HCO3- transport, intracellular CO2 and HCO3- accumulation, and photosynthesis in the presence but not in the absence of 25 mM Na+. These activities returned to near-normal levels within 15 min. Transient inhibition was attributed to monensin-mediated intracellular alkalinization, whereas recovery may have been facilitated by cellular mechanisms involved in pH homeostasis or by monensin-mediated H+ uptake with concomitant K+ efflux. In air-grown cells grown at 200 [mu]M CO2 + HCO3- and standing culture cells, Na+-dependent HCO3- transport, intracellular HCO3- accumulation, and photosynthesis were also inhibited by monensin, but there was little recovery in activity over time. However, normal photosynthetic activity could be restored to air-grown cells by the addition of carbonic anhydrase, which increased the rate of CO2 supply to the cells. This observation indicated that of all the processes required to support photosynthesis only Na+-dependent HCO3- transport was significantly inhibited by monensin. Monensin-mediated dissipation of the Na+ chemical gradient between the medium and the cells largely accounted for the decline in the HCO3- accumulation ratio from 751 to 55. The two HCO3- transport systems were further distinguished in that Na+-dependent HCO3- transport was inhibited by Li+, whereas Na+-independent HCO3- transport was not. It is suggested that Na+-dependent HCO3- transport involves an Na+/HCO3- symport mechanism that is energized by the Na+ electrochemical potential.
莫能菌素是一种介导Na⁺/H⁺交换的离子载体，利用基于叶绿素a荧光发射测量或¹⁴C摄取的转运测定法，研究了其对蓝藻聚球藻UTEX 625无机碳转运系统活性的影响。在约20 μM CO₂ + HCO₃⁻的静置培养条件下生长的聚球藻细胞中，50 μM莫能菌素在有25 mM Na⁺存在时会短暂抑制活性CO₂和不依赖Na⁺的HCO₃⁻转运、细胞内CO₂和HCO₃⁻积累以及光合作用，但在无Na⁺时则不会。这些活性在15分钟内恢复到接近正常水平。短暂抑制归因于莫能菌素介导的细胞内碱化，而恢复可能是由参与pH稳态的细胞机制促进的，或者是由莫能菌素介导的H⁺摄取伴随K⁺外流促进的。在200 μM CO₂ + HCO₃⁻条件下生长的气生细胞和静置培养细胞中，依赖Na⁺的HCO₃⁻转运、细胞内HCO₃⁻积累和光合作用也受到莫能菌素的抑制，但随着时间推移活性几乎没有恢复。然而，通过添加碳酸酐酶可以使气生细胞恢复正常光合活性，这增加了细胞的CO₂供应速率。这一观察结果表明，在支持光合作用所需的所有过程中，只有依赖Na⁺的HCO₃⁻转运受到莫能菌素的显著抑制。莫能菌素介导的培养基与细胞之间Na⁺化学梯度消散在很大程度上导致了HCO₃⁻积累率从751降至55。这两种HCO₃⁻转运系统的进一步区别在于，依赖Na⁺的HCO₃⁻转运受到Li⁺抑制，而不依赖Na⁺的HCO₃⁻转运不受影响。有人提出，依赖Na⁺的HCO₃⁻转运涉及一种由Na⁺电化学势供能的Na⁺/HCO₃⁻同向转运机制。
