Furesz T C, Moe A J, Smith C H
Department of Pediatrics, Washington University School of Medicine, St. Louis Children's Hospital, Missouri 63110.
Am J Physiol. 1991 Aug;261(2 Pt 1):C246-52. doi: 10.1152/ajpcell.1991.261.2.C246.
Transport of cationic amino acids in basal (fetal facing) plasma membranes was investigated by characterization of L-[3H]lysine and L-[3H]arginine uptake in membrane vesicles isolated from term human placentas. At least two Na(+)-independent systems were present. Lysine concentration dependence data were fit by a two-system model with Km values of 1.0 +/- 0.8 and 223 +/- 57 microM and Vmax values of 0.06 +/- 0.03 and 24.0 +/- 5.8 pmol.mg protein-1.min-1. In the presence of either 10 mM L-leucine or Na+ plus 10 mM L-homoserine, the data were fit by single system models with kinetic parameters similar to the higher and lower Km systems seen in the absence of inhibitors. Uptake of 10 or 20 microM L-lysine in the absence of Na+ showed the higher Km system was inhibited completely by L-arginine, L-homoarginine, and L-histidine. In the presence of Na+, the higher Km system was inhibited completely by L-alanine, L-homoserine, L-leucine, L-phenylalanine, and L-norleucine. The lower Km system was inhibited completely by L-arginine, L-homoarginine, L-histidine, L-leucine, and L-methionine. Time course studies of uptake demonstrated that uptake by either system alone filled the total vesicular space. The basal membrane of human placental syncytiotrophoblast possesses two transport systems for lysine and arginine, resembling the ubiquitous y+ system and the bo,+ system previously described in mouse blastocysts. The higher Vmax of the y+ system suggests that in utero it may mediate transfer of cationic amino acids from the syncytiotrophoblast to the fetus. The role of the high-affinity low-capacity bo,+ system remains to be determined.
通过对足月人胎盘分离的膜囊泡中L-[3H]赖氨酸和L-[3H]精氨酸摄取的特性分析，研究了阳离子氨基酸在基底（面向胎儿）质膜中的转运。至少存在两个不依赖Na(+)的系统。赖氨酸浓度依赖性数据通过双系统模型拟合，Km值分别为1.0±0.8和223±57μM，Vmax值分别为0.06±0.03和24.0±5.8 pmol·mg蛋白-1·min-1。在存在10 mM L-亮氨酸或Na+加10 mM L-高丝氨酸的情况下，数据通过单系统模型拟合，其动力学参数类似于在无抑制剂时观察到的较高和较低Km系统。在无Na+时，10或20μM L-赖氨酸的摄取显示较高Km系统被L-精氨酸、L-高精氨酸和L-组氨酸完全抑制。在有Na+时，较高Km系统被L-丙氨酸、L-高丝氨酸、L-亮氨酸、L-苯丙氨酸和L-正亮氨酸完全抑制。较低Km系统被L-精氨酸、L-高精氨酸、L-组氨酸、L-亮氨酸和L-蛋氨酸完全抑制。摄取的时间进程研究表明，单独由任一系统摄取都充满了整个囊泡空间。人胎盘合体滋养层基底膜拥有两个赖氨酸和精氨酸转运系统，类似于先前在小鼠胚泡中描述的普遍存在的y+系统和bo,+系统。y+系统较高的Vmax表明，在子宫内它可能介导阳离子氨基酸从合体滋养层向胎儿的转运。高亲和力低容量bo,+系统的作用尚待确定。
