Garvey T Q, Babcock A
J Membr Biol. 1979 Aug;49(2):139-56. doi: 10.1007/BF01868722.
The studies reported here were carried out to characterize further previously described changes in membrane localized amino acid transport associated with simian virus 40 transformation of the mammalian cell line, Balb/c3T3. Membrane vesicles were prepared from confluent cultures of both simian virus 40 transformed Balb/c3T3 (SV3T3) and the untransformed parent line, Balb/c3T3 (3T3). An initial, externally imposed out greater than in, 100 mM a+ gradient produces acceleration of early ingress of alpha-aminoisobutyric acid (AlB) in vesicles from both cell lines, but transient, concentrative uptake (overshooting) only in SV3T3 vesicles. Early ingress of L-leucine is also accelerated in SV3T3 vesicles by a Na+ gradient, and overshooting is also demonstrable. Na+-gradient independent AIB permeability of SV3T3 and 3T3 membranes was estimated using uptake data, a first order rate equation and measurements of vesicle size derived from quasi-elastic light-scattering studies. AIB permeability of SV3T3 membranes is greater than that of 3T3 membranes (113 A/min and 43 A/min, respectively), suggesting that overshooting in 3T3 vesicles is not attenuated by a Na+-independent AIB "leak". Na+ permeability of the two membranes is similar, ruling out the possibility that a slower rate of Na+ equilibration across the SV3T3 membrane allows development of the overshoot. In SV3T3 vesicles the height of a Na+-gradient dependent overshoot varies with the initial [Na+]o/[Na+]i ratio, and ln[Na+]o/[Na+]i is linearly related to ln AIB uptake at overshoot peak/AIB uptake at equilibrium, consistent with the possibility that for [Na+]o/[Na+]i ratios in the range studied, AIB overshoot is energized by a constant proportion of the energy available from the initial electrochemical gradient for Na+. These results are consistent with the possibility that Na+-gradient dependent overshooting in SV3T3 vesicles is produced by Na+-amino acid carrier interactions resulting in either an increase in maximum transport velocity or an increase in carrier affinity for AIB.
本文报道的研究旨在进一步表征先前描述的与猿猴病毒40转化哺乳动物细胞系Balb/c3T3相关的膜定位氨基酸转运变化。从猿猴病毒40转化的Balb/c3T3（SV3T3）和未转化的亲代细胞系Balb/c3T3（3T3）的汇合培养物中制备膜囊泡。最初，外部施加的细胞外大于细胞内、100 mM的Na⁺梯度会加速两种细胞系囊泡中α-氨基异丁酸（AIB）的早期进入，但仅在SV3T3囊泡中出现短暂的浓缩摄取（超射）。Na⁺梯度也会加速SV3T3囊泡中L-亮氨酸的早期进入，并且超射也可被证实。使用摄取数据、一级速率方程以及通过准弹性光散射研究得出的囊泡大小测量值，估算了SV3T3和3T3膜对Na⁺梯度无关的AIB通透性。SV3T3膜的AIB通透性大于3T3膜（分别为113 Å/min和43 Å/min），这表明3T3囊泡中的超射不会因Na⁺无关的AIB“泄漏”而减弱。两种膜的Na⁺通透性相似，排除了跨SV3T3膜较慢的Na⁺平衡速率导致超射出现的可能性。在SV3T3囊泡中，Na⁺梯度依赖性超射的高度随初始[Na⁺]o/[Na⁺]i比值而变化，并且ln[Na⁺]o/[Na⁺]i与超射峰值时的ln AIB摄取量/平衡时的AIB摄取量呈线性相关，这与在所研究的[Na⁺]o/[Na⁺]i比值范围内，AIB超射由初始Na⁺电化学梯度中可用能量的恒定比例提供能量的可能性一致。这些结果与以下可能性一致，即SV3T3囊泡中Na⁺梯度依赖性超射是由Na⁺-氨基酸载体相互作用产生的，这导致最大转运速度增加或载体对AIB的亲和力增加。
