Characterization of ammonia transport by the kidney Rh glycoproteins RhBG and RhCG
Characterization of ammonia transport by the kidney Rh glycoproteins RhBG and RhCG
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DOI：
10.1152/ajprenal.00147.2005
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发表时间：
2006-02-01
影响因子：
4.2
通讯作者：
Westhoff, CM
中科院分区：
文献类型：
--
作者：
Mak, DOD;Dang, B;Westhoff, CM
红细胞Rh相关糖蛋白（RhAG）在非洲爪蟾卵母细胞和酵母酿酒酵母中表达，可介导氨/铵的转运。非红系同源物RhBG和RhCG在哺乳动物肾连接段和集合管中表达，这是尿氨分泌的主要部位。本研究通过[（14）C]甲胺（MA）摄取和X.卵母细胞RhBG和RhCG都介导氨的转运，但对底物的亲和力不同（Km分别为2.5和10 mM）。RhBG-和RhCG-介导的转运速率对质子化MA物质的浓度敏感，并受到细胞外代谢的刺激和酸中毒的抑制，表明H（+）在转运过程中的作用。而RhBG或RhCG的表达引起质膜电导的小幅增加，[14 C] MA摄取不受卵母细胞的去极化与100 mM的细胞外K（+）或钳膜电位在0和-100 mV之间的影响，表明RhBG-和RhCG-介导的运输是独立的膜电位。这些结果强烈地表明RhBG和RhCG通过电中性过程转运氨，该过程涉及NH（4）（+）/H（+）交换，导致净NH（3）转运。RhBG和RhCG在肾小管中的极化定位和不同的底物亲和力可能使这些蛋白质参与跨上皮氨分泌，因此在整个动物的酸碱调节中发挥重要作用。
The erythrocyte Rh-associated glycoprotein (RhAG) was recently found to mediate transport of ammonia/ammonium when expressed in Xenopus laevis oocytes and yeast Saccharomyces cerevisiae. Nonerythroid homologs, RhBG and RhCG, are expressed in the mammalian kidney connecting segment and the collecting duct, major sites of urinary ammonia secretion. This study characterizes the transport properties of murine RhBG and RhCG by ammonium analog [(14)C] methylamine (MA) uptake and two-electrode voltage clamping of X. laevis oocytes. Both RhBG and RhCG mediated transport of ammonia, but differed in affinity for substrate (Km = 2.5 and 10 mM, respectively). The rates of RhBG- and RhCG- mediated transport were sensitive to the concentration of the protonated MA species and were stimulated by extracellular alkalosis and inhibited by acidosis, suggesting a role for H(+) in the transport process. Whereas expression of RhBG or RhCG caused a small increase in plasma membrane conductance, [14C] MA uptake was not affected by depolarization of oocytes with 100 mM extracellular K(+) or by clamping the membrane potential between 0 and -100 mV, indicating that RhBG- and RhCG- mediated transport was independent of the membrane potential. These results strongly suggest that RhBG and RhCG transport ammonia by an electroneutral process that involves NH(4)(+)/H(+) exchange resulting in net NH(3) translocation. The polarized localization of RhBG and RhCG in kidney tubules and the different substrate affinities may enable these proteins to participate in transepithelial ammonia secretion and to therefore play an important role in whole animal acid-base regulation.