Van Winkle L J, Campione A L, Gorman J M
Department of Biochemistry, Chicago College of Osteopathic Medicine, Downers Grove, IL 60515.
Biochim Biophys Acta. 1990 Jun 27;1025(2):215-24. doi: 10.1016/0005-2736(90)90100-3.
The most conspicuous, Na(+)-independent amino acid transport process in preimplantation mouse blastocysts was provisionally designated system b0,+ because it accepts some cationic and zwitterionic amino acids about equally well as substrates. Although system b0,+ is not Na(+)-stimulated, it has not been determined if it is inhibited by Na+, or if its activity is affected by most other ions. Therefore, we measured uptake of amino acids by blastocysts in isotonic solutions of different ionic and nonionic osmolites. Na(+)-independent L-leucine uptake was unaffected by the ion concentration, but L-lysine transport was several-fold faster in isotonic solutions of non-electrolytes than in similar solutions of inorganic and organic salts or zwitterions. The Km value for 'Na(+)-independent' L-lysine transport was about 10-fold higher in isotonic salt solutions than in solutions of nonionic osmolites, whereas the Km value for L-leucine transport was about the same in either type of solution. Therefore, inorganic and organic cations and the cationic portions of zwitterions appear to compete with cationic but not zwitterionic amino acids for system b0,+ receptor sites. The cation, harmaline, was a particularly strong competitive inhibitor of 'Na(+)-independent' L-lysine uptake by system b0,+, even in isotonic salt solutions, whereas it inhibited L-leucine uptake noncompetitively. Moreover, harmaline appeared to compete with inorganic cations for the lysine receptor sites of system b0,+. Harmaline also has been found by other investigators to competitively inhibit L-lysine uptake by the Na(+)-independent system asc1 in horse erythrocytes, whereas it noncompetitively inhibits alanine uptake by the same system. Similarly, harmaline noncompetitively inhibits L-alanine uptake by the Na(+)-dependent system ASC in human erythrocytes, but it appears to compete for binding with L-alanine's cosubstrate, Na+. In addition, others have found that the positively-charged side chains of cationic amino acids seem to take the place of Na+ needed near side chains in order for zwitterionic amino acids to be transported by systems ASC and y+. We conclude that system b0,+ may be similar to systems asc1, ASC and y+, and that each of these systems may be a variant of the same ancestral transport process. We speculate that since it appears to accept a broader scope of substrates and to interact with a wider variety of cations than do systems asc1, ASC or y+, system b0,+ may more closely resemble the proposed ancestral transport process than any of the other contemporary systems.
在植入前的小鼠囊胚中，最显著的、不依赖钠离子的氨基酸转运过程被暂时命名为系统b0,+，因为它对一些阳离子和两性离子氨基酸的接纳能力大致相同，均可作为底物。尽管系统b0,+不受钠离子刺激，但尚未确定它是否会被钠离子抑制，或者其活性是否会受到大多数其他离子的影响。因此，我们在不同离子和非离子渗透剂的等渗溶液中测量了囊胚对氨基酸的摄取。不依赖钠离子的L-亮氨酸摄取不受离子浓度影响，但在非电解质等渗溶液中，L-赖氨酸的转运速度比在类似的无机盐、有机盐或两性离子溶液中快几倍。在等渗盐溶液中，“不依赖钠离子”的L-赖氨酸转运的Km值比在非离子渗透剂溶液中高约10倍，而L-亮氨酸转运的Km值在这两种类型的溶液中大致相同。因此，无机和有机阳离子以及两性离子的阳离子部分似乎与阳离子氨基酸而非两性离子氨基酸竞争系统b0,+的受体位点。阳离子哈尔满是系统b0,+介导的“不依赖钠离子”的L-赖氨酸摄取的一种特别强的竞争性抑制剂，即使在等渗盐溶液中也是如此，而它对L-亮氨酸摄取的抑制是非竞争性的。此外，哈尔满似乎与无机阳离子竞争系统b0,+的赖氨酸受体位点。其他研究人员还发现，哈尔满竞争性抑制马红细胞中不依赖钠离子的系统asc1对L-赖氨酸的摄取，而对同一系统介导的丙氨酸摄取的抑制是非竞争性的。同样，哈尔满对人红细胞中依赖钠离子的系统ASC介导的L-丙氨酸摄取的抑制是非竞争性的，但它似乎与L-丙氨酸的共底物钠离子竞争结合位点。此外，其他人发现，阳离子氨基酸带正电荷的侧链似乎取代了两性离子氨基酸通过系统ASC和y+转运时侧链附近所需的钠离子。我们得出结论，系统b0,+可能与系统asc1、ASC和y+相似，并且这些系统中的每一个可能都是同一祖先转运过程的变体。我们推测，由于系统b0,+似乎比系统asc1、ASC或y+能接纳更广泛的底物范围并与更多种类的阳离子相互作用，它可能比任何其他当代系统更接近所提出的祖先转运过程。
