Servaites Jerome C, Geiger Donald R
Department of Biology, University of Dayton, Dayton, OH 45469-2320, USA.
J Exp Bot. 2002 Jul;53(374):1581-91. doi: 10.1093/jxb/erf009.
Influx of labelled D-glucose into isolated spinach (Spinacia oleracea L. cv. Melody hybrid) chloroplasts was initially rapid followed by a period of slower influx. The stroma glucose concentration attained equilibrium rapidly with low external glucose concentrations and the two were linearly proportional. The period of slower influx resulted from conversion of glucose to acidic products that remained trapped in the chloroplast. As the external glucose concentration increased, the stroma glucose concentration increased less and less, attaining a maximal concentration of 72 mol m(-3). The maintenance of an equilibrium stroma glucose concentration lower than that in the external medium is evidence that plastid glucose efflux involves secondary active transport. The equilibrium stroma glucose concentration increased in response to light and protonophoric uncouplers. It is proposed that glucose efflux is coupled with a proton and the stroma glucose concentration equilibrates in response to the proton gradient across the membrane. To determine if glucose is a significant product of starch mobilization, chloroplasts were isolated from spinach leaves labelled with 14CO2 during the preceding light period. Chloroplasts degraded starch at the same rate as the intact leaf. Glucose, maltose, and isomaltose were the principal labelled products that appeared in the medium during starch mobilization. The glucose concentration in the chloroplast was 2 mol m(-3), which is similar to the measured Km for zero trans efflux. The data support the role of the glucose translocator as an important component in the pathway for sucrose synthesis at night.
标记的D-葡萄糖流入分离的菠菜（菠菜品种旋律杂交种）叶绿体最初很快，随后是一段较慢的流入期。在低外部葡萄糖浓度下，基质葡萄糖浓度迅速达到平衡，两者呈线性比例关系。流入较慢的时期是由于葡萄糖转化为仍被困在叶绿体中的酸性产物所致。随着外部葡萄糖浓度的增加，基质葡萄糖浓度的增加越来越少，达到最大浓度72 mol m(-3)。维持低于外部介质的基质葡萄糖平衡浓度证明质体葡萄糖外流涉及次级主动运输。基质葡萄糖平衡浓度响应光照和质子载体解偶联剂而增加。有人提出葡萄糖外流与质子偶联，并且基质葡萄糖浓度响应跨膜质子梯度而达到平衡。为了确定葡萄糖是否是淀粉动员的重要产物，从先前光照期间用14CO2标记的菠菜叶中分离叶绿体。叶绿体降解淀粉的速度与完整叶片相同。葡萄糖、麦芽糖和异麦芽糖是淀粉动员期间培养基中出现的主要标记产物。叶绿体中的葡萄糖浓度为2 mol m(-3)，这与零转运外流的测量Km相似。这些数据支持葡萄糖转运体作为夜间蔗糖合成途径中重要组成部分的作用。
