Furesz T C, Smith C H
Edward Mallinckrodt Department of Pediatrics, St Louis Children's Hospital, Washington University School of Medicine, Missouri 63110, USA.
Placenta. 1997 Nov;18(8):649-55. doi: 10.1016/s0143-4004(97)90006-0.
The recent demonstration of multiple high-affinity leucine-sensitive cationic transport systems prompted this investigation of their role in lysine uptake in basal cell membrane. Transport of lysine by basal membrane was saturable at both 22 and 37 degrees C and linear in time to 1 min and 30 sec, respectively. At 22 degrees C, at least two systems were active. The portion of uptake inhibited by the sulphydryl binding reagent N-ethylmaleimide (NEM) but not by leucine in the absence of sodium had a high K(m) and high Vmax and was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) (+/- s.e.) of 4 +/- 1 microM and a Vmax of 0.9 +/- 0.1 pmol/mg protein/min. This component was completely inhibited by leucine in the absence of sodium but not by glutamine in the presence of sodium. Therefore, it was attributed to system bo,+. At 37 degrees C, at least three systems were active. For essentially the same reasons as above the NEM inhibitable uptake was attributed to system y+. NEM-insensitive uptake was fitted by a one-system model with K(m) of 26 +/- 7 microM and Vmax of 11.1 +/- 2.8 pmol/mg protein/30 sec. Inhibition studies, however, indicated its heterogeneity. NEM-insensitive saturable uptake was only partially inhibited by either leucine in the absence of sodium (system bo,+) or by glutamine in the presence of sodium (system y+L). It is concluded that the NEM-insensitive portion of lysine uptake at 37 degrees C represents activity of both system bo,+ and the temperature-sensitive system y+L. As a previous investigation indicates, only one of these (system y+L) is present in the more specialized microvillous membrane. The demonstration of functional differences in the high affinity leucine transporters of basal and microvillous membrane in this and our previous investigations suggest that the two membranes possess different transport or modifier proteins.
最近对多种高亲和力亮氨酸敏感阳离子转运系统的证明促使了对其在基底细胞膜赖氨酸摄取中作用的这项研究。在22℃和37℃时，基底膜对赖氨酸的转运均具有饱和性，且在时间上分别在1分钟和30秒内呈线性。在22℃时，至少有两个系统是活跃的。巯基结合试剂N - 乙基马来酰亚胺（NEM）抑制但在无钠情况下亮氨酸不抑制的摄取部分具有高K(m)和高Vmax，归因于系统y+。NEM不敏感的摄取由一个单系统模型拟合，K(m)（±标准误）为4±1微摩尔，Vmax为0.9±0.1皮摩尔/毫克蛋白/分钟。该组分在无钠情况下被亮氨酸完全抑制，但在有钠情况下不被谷氨酰胺抑制。因此，它归因于系统bo,+。在37℃时，至少有三个系统是活跃的。出于与上述基本相同的原因，NEM可抑制的摄取归因于系统y+。NEM不敏感的摄取由一个单系统模型拟合，K(m)为26±7微摩尔，Vmax为11.1±2.8皮摩尔/毫克蛋白/30秒。然而，抑制研究表明其具有异质性。NEM不敏感的饱和摄取仅部分被无钠情况下的亮氨酸（系统bo,+）或有钠情况下的谷氨酰胺（系统y+L）抑制。得出的结论是，37℃时赖氨酸摄取的NEM不敏感部分代表系统bo,+和温度敏感系统y+L的活性。如先前的一项研究所示，这些系统中只有一个（系统y+L）存在于更特殊的微绒毛膜中。在本研究和我们先前的研究中，基底膜和微绒毛膜的高亲和力亮氨酸转运体功能差异的证明表明，这两种膜具有不同的转运或调节蛋白。
