Selenium and Sulfur Amino Acid Derivatives

l-Glutamine, l-methionine, l-selenomethionine (SM), S-methyl-l-cysteine (SMC), sodium α-keto-γ-methiolbutyrate (KMB; 4-methylthio-2-oxobutanoate), sodium α-ketoglutarate, β-mercaptoethanol, dithiothreitol (DTT), sodium dodecyl sulfate (SDS) and metaphosphoric acid (MPA) were obtained from Sigma-Aldrich (St. Louis, MO, USA). The α-keto acids derived from SM and SMC (i.e., α-keto-γ-methylselenobutyrate (KMSB) and β-methylselenopyruvate (MSP), respectively) were made in situ from the corresponding l-amino acids with snake venom l-amino acid oxidase [10 (link)]. 2,4-Dinitrophenylhydrazine was obtained from Kodak (Rochester, NY, USA). KGM was synthesized from l-glutamine with l-amino acid oxidase as described [11 (link)].