D'Ambrosio S M, Glover G I, Nelson S O, Jensen R A
J Bacteriol. 1973 Aug;115(2):673-81. doi: 10.1128/jb.115.2.673-681.1973.
l-Tyrosine and l-phenylalanine enter cells of Bacillus subtilis via a system of active transport that exhibits complex kinetic behavior. The specificity of the transport system was characterized both at low concentrations of transport substrate (where affinity for l-tyrosine or l-phenylalanine is high but capacity is low) and at high concentrations (where affinity is low but capacity is high). Specificity was not found to differ significantly as a function of either l-tyrosine or l-phenylalanine concentration. Kinetic analysis showed that the relationship between the uptake of l-phenylalanine and l-tyrosine is strictly competitive. Neither l-tyrosine nor l-phenylalanine uptake was competitively inhibited by other naturally occurring l-amino acids, indicating the importance of the phenyl side chain to uptake specificity. Hence, it is concluded that l-tyrosine and l-phenylalanine are transported by a common system that is specific for these two amino acids. The abilities of analogue derivatives of l-tyrosine and l-phenylalanine to inhibit the uptake of l-[(14)C]tyrosine and l-[(14)C]phenylalanine competitively were determined throughout a wide range of substrate and inhibitor concentrations. In this manner, the contributions of the side chain, the alpha-amino group and the carboxyl group to uptake specificity were established. It is concluded that the positively charged alpha-amino group contributes more significantly to uptake specificity than does the negatively charged carboxyl group. The recognition of a phenyl ring is an essential feature of specificity; other amino acids with aromatic side chains, such as the indole and imidazole rings of l-tryptophan and l-histidine, do not compete with l-tyrosine and l-phenylalanine for uptake. The presence of the p-hydroxy substitutent in the side chain (as in l-tyrosine) enhances the uptake of the aryl amino acid analogues investigated.
L-酪氨酸和L-苯丙氨酸通过一个表现出复杂动力学行为的主动运输系统进入枯草芽孢杆菌细胞。在低浓度运输底物（对L-酪氨酸或L-苯丙氨酸亲和力高但转运能力低）和高浓度（亲和力低但转运能力高）条件下，均对该运输系统的特异性进行了表征。未发现特异性随L-酪氨酸或L-苯丙氨酸浓度的变化而有显著差异。动力学分析表明，L-苯丙氨酸和L-酪氨酸摄取之间的关系是严格竞争性的。L-酪氨酸和L-苯丙氨酸的摄取均未受到其他天然存在的L-氨基酸的竞争性抑制，这表明苯侧链对摄取特异性的重要性。因此，可以得出结论，L-酪氨酸和L-苯丙氨酸是由一个对这两种氨基酸具有特异性的共同系统进行运输的。在广泛的底物和抑制剂浓度范围内，测定了L-酪氨酸和L-苯丙氨酸类似物竞争性抑制L-[(14)C]酪氨酸和L-[(14)C]苯丙氨酸摄取的能力。通过这种方式，确定了侧链、α-氨基和羧基对摄取特异性的贡献程度。得出的结论是，带正电荷的α-氨基对摄取特异性的贡献比带负电荷的羧基更为显著。苯环的识别是特异性的一个基本特征；其他带有芳香侧链的氨基酸，如L-色氨酸和L-组氨酸的吲哚环和咪唑环，不会与L-酪氨酸和L-苯丙氨酸竞争摄取。侧链中对羟基取代基（如L-酪氨酸中的）的存在增强了所研究的芳基氨基酸类似物的摄取。
