Reinhold L, Eshhar Z
Department of Botany, Hebrew University of Jerusalem, Israel.
Plant Physiol. 1968 Jul;43(7):1023-30. doi: 10.1104/pp.43.7.1023.
The movement of labeled 3-o-methylglucose (MeG) into and out of thin carrot discs has been followed in order to gain information on sugar entry and exit mechanisms. Little or no metabolism of this derivative appears to occur in the tissue, since no products were detected either by chromatography or by analysis of respiratory CO(2).The curve relating entry to external concentration deviated somewhat from a rectangular hyperbola but suggested a carrier mechanism. Glucose and MeG each competitively inhibited the uptake of the other. K(i) for MeG was estimated to be 3 times the K(m) for its uptake.When discs incubated in MeG were transferred to H(2)O, MeG lost to the solution from the Free Space was re-absorbed against a 7-fold concentration gradient.The addition of unlabeled MeG or glucose to the medium surrounding discs which had been maintaining a ratio of internal to external MeG of 75:1 brought about release of stored isotope. This was probably not due to exchange diffusion stricto sensu.Efflux of previously absorbed isotopic MeG into a medium containing unlabeled MeG or glucose was temperature-sensitive. The kinetics of efflux were complex and did not suggest a simple diffusion process related to overall MeG content. However there is evidence (including the falling rate of exit with time) that slow diffusion (or slow release from adsorption) contributed substantially to efflux. The source of this flow appeared to be neither the readily accessible Free Space nor the main storage compartment. Calculation indicated that the volume of this "slow diffusion compartment" might be about 1% of the total volume of the discs.
为了获取有关糖类进出机制的信息，研究了标记的3 - O - 甲基葡萄糖（MeG）进出胡萝卜薄片的运动情况。由于通过色谱法或呼吸CO₂分析均未检测到产物，所以该衍生物在组织中似乎很少或几乎没有代谢。与外部浓度相关的进入曲线与矩形双曲线略有偏差，但表明存在载体机制。葡萄糖和MeG相互竞争性抑制对方的摄取。MeG的抑制常数（K(i)）估计是其摄取的米氏常数（K(m)）的3倍。当在MeG中孵育的薄片转移到水中时，从自由空间流失到溶液中的MeG会逆着7倍的浓度梯度被重新吸收。向维持内部与外部MeG比例为75:1的薄片周围培养基中添加未标记的MeG或葡萄糖会导致储存同位素的释放。这可能并非严格意义上的交换扩散所致。先前吸收的同位素MeG向含有未标记MeG或葡萄糖的培养基中的流出对温度敏感。流出动力学很复杂，并不表明是与整体MeG含量相关的简单扩散过程。然而，有证据（包括随着时间推移流出速率下降）表明缓慢扩散（或从吸附中缓慢释放）对流出有很大贡献。这种流动的来源似乎既不是易于接近的自由空间，也不是主要的储存区室。计算表明，这个“缓慢扩散区室”的体积可能约为薄片总体积的1%。
