Fukumoto G H, Byus C V
Department of Biochemistry, University of California, Riverside 92521-0121, USA.
Biochim Biophys Acta. 1996 Jun 13;1282(1):48-56. doi: 10.1016/0005-2736(96)00036-3.
Using human erythrocytes as a model system for the study of mammalian polyamine transport, detailed kinetic parameters regarding the uptake and export of putrescine and spermidine were determined. The putrescine uptake data indicated a multi-component uptake system comprised of a low-capacity saturable component and a non-saturable component. The saturable putrescine uptake component demonstrated a calculated Km of 21.0 microM and a V(max) of only 6.52 x 10(-13) M/s. The non-saturable linear putrescine uptake rate was defined by a significant pH dependence, a lack of uptake inhibition by related polyamines, and a permeability pi of 3.19 x 10(-8) s-1. These findings suggested that non-saturable putrescine uptake involved a process of simple diffusion. Spermidine uptake exhibited Michaelis-Menten kinetics with a Km and Vmax of 12.5 microM and 1.36 x 10(-12) M/s, respectively. Spermidine uptake did not demonstrate pH dependence and was not significantly inhibited by any of the tested polyamines. The Arrhenius plot of spermidine uptake was determined to be biphasic with calculated activation energies of spermidine uptake of 135.2 kJ/mol for 19-21 degrees C and 59.3 kJ/mol for 21-35 degrees C. These data suggest the possibility of multiple spermidine uptake processes which are not mediated by simple diffusion across the cell membrane. The putrescine export process demonstrated both saturable and non-saturable components. The calculated Km, V(max) and pi for putrescine export were 33.8 microM, 1.19 x 10(-11) M/s and 2.81 x 10(-7) s-1, respectively. The spermidine export process was non-saturable up to intracellular spermidine concentrations of 4 microM. At similar intracellular and extracellular concentrations of putrescine and spermidine, however, export processes displayed rates which were an order of magnitude greater than their respective uptake rates. This finding supports the possible presence of mediated putrescine and spermidine export processes different than simple diffusion.
以人红细胞作为研究哺乳动物多胺转运的模型系统，测定了有关腐胺和亚精胺摄取与输出的详细动力学参数。腐胺摄取数据表明其摄取系统由一个低容量可饱和组分和一个非饱和组分组成。可饱和的腐胺摄取组分的计算出的米氏常数（Km）为21.0微摩尔，最大反应速率（V(max)）仅为6.52×10⁻¹³摩尔/秒。非饱和的线性腐胺摄取速率的特点是对pH有显著依赖性、不受相关多胺的摄取抑制，通透系数（pi）为3.19×10⁻⁸秒⁻¹。这些发现表明非饱和的腐胺摄取涉及简单扩散过程。亚精胺摄取表现出米氏动力学，Km和Vmax分别为12.5微摩尔和1.36×10⁻¹²摩尔/秒。亚精胺摄取不表现出对pH的依赖性，也不受任何测试多胺的显著抑制。亚精胺摄取的阿伦尼乌斯曲线被确定为双相的，在19至21摄氏度时亚精胺摄取的计算活化能为135.2千焦/摩尔，在21至35摄氏度时为59.3千焦/摩尔。这些数据表明存在多种亚精胺摄取过程的可能性，这些过程不是通过简单扩散穿过细胞膜介导的。腐胺输出过程显示出可饱和和非饱和组分。腐胺输出的计算出的Km、V(max)和pi分别为33.8微摩尔、1.19×¹⁰⁻¹¹摩尔/秒和2.81×10⁻⁷秒⁻¹。亚精胺输出过程在细胞内亚精胺浓度达到4微摩尔之前是非饱和的。然而，在腐胺和亚精胺的细胞内和细胞外浓度相似时，输出过程的速率比它们各自的摄取速率高一个数量级。这一发现支持可能存在不同于简单扩散的介导的腐胺和亚精胺输出过程。
