ABSTRACT The high stereo- and substrate specificities of enzymes have been utilized for microdetermination of amino acids. Here, I review the discovery of l-Arg oxidase from Pseudomonas sp. TPU 7192, l-Lys oxidase/decarboxylase from Burkholderia sp. AIU 395, and enzymes showing apparent l-His oxidase activity from Achromobacter sp. TPU 5009. I also discuss screening and uses of the selective enzymes for microdetermination of amino acids. In addition, functional modifications of l-amino acid oxidase/monooxygenase from Pseudomonas sp. AIU 813, l-Trp dehydrogenase from Nostoc punctiforme ATCC 29133, and l-Lys ε-oxidase from Marinomonas mediterranea NBRC 103028 by directed evolution are reviewed. Finally, I review the rational identification of aggregation hotspots based on secondary structure and amino acid hydrophobicity—this process enables the wider use of natural enzymes.