L alanine

Samples were obtained from commercial sources as follows: l-aspartic acid sodium salt, l-glutamic acid monosodium salt, l-serine, l-lysine monohydrochloride, l-histidine monohydrochloride monohydrate, l-proline, and l-phenylalanine were purchased from Nacalai Tesque; l-glutamine, l-threonine, glycine, l-alanine, l-valine, l-isoleucine, l-leucine, l-arginine, and l-asparagine monohydrate were obtained from Kanto Chemical; l-methionine, methionol, pentanal, 3-(methylthio)propionic acid, and 3-(methylthio)butanal were obtained from Tokyo Chemical Industry; methional was from Sigma Aldrich; 2-methylthioacetaldehyde was from Penta Manufacturing Company; 3-(ethylthio)propanal was from Enamine Ltd.; 4-(methylthio)butanal was from FCH Group; and coelenterazine was purchased from Promega.

The assay buffer (referred to as high Cl⁻ buffer) was composed of 10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 130 mM NaCl, 10 mM glucose, 5 mM KCl, 2 mM CaCl₂, and 1.2 mM MgCl₂. The buffers with lower Cl⁻ were prepared by progressively replacing NaCl, KCl, CaCl₂ and MgCl₂ in the buffer with gluconate equivalents^{20 (link)}. The pH was adjusted to 7.4 with NaOH. Ligands were diluted with the assay buffers at the desired concentrations.
Amino acids were obtained from commercial sources as follows. L-proline was purchased from Nacalai Tesque (Kyoto, Japan); L-alanine and L-glutamine were purchased from Kanto Chemical (Tokyo, Japan). Sucralose and sodium N-cyclohexyl-sulfamate (cyclamate) were purchased from Sigma–Aldrich Japan (Tokyo, Japan). D-tryptophan was purchased from Yoneyama Yakuhin Kogyo (Osaka, Japan).