Amino acids, then and now--a reflection on Sir Hans Krebs' contribution to nitrogen metabolism.

H. A. Krebs made an enormous contribution to our knowledge of amino acid metabolism, beginning with his studies on proteolysis in the early 1930s, progressing through his work on urea synthesis to an extensive series of papers on deamination and, then, to work on gluconeogenesis from amino acids. This paper addresses three of Krebs' early contributions-urea synthesis, glutamine metabolism, and D-amino acid oxidase-and relates them to our modern understanding of amino acid metabolism.