role of Adenosine 5'-triphosphate (ATP) as a common metabolic "energy carrier." The paper is not highly cited, most likely, because Kalckar's mentor and friend, Fritz Lipmann, published a similar review in which he introduced the notion of "~P" as a means of representing a "high energy" bond. The onset of World War II stranded the Kalckars in the United States; Kalckar was offered an appointment as a research fellow at Washington University, and he and his wife moved to St. Louis. Kalckar resumed his friendship with Colowick, and they collaborated to work on the enzyme adenylate kinase in 1942, which they purified from muscle extracts. Further work on nucleotide metabolism allowed him to identify nucleoside phosphorylase, a key enzyme in nucleotide salvage pathways. The work was very important in expanding Kalckar's research interests because previously he had worked with complex physiological systems. In his Washington University work, he focused on individual enzymes and their purification. At an invitation from Oliver Lowry (1910–1996), Kalckar moved to the New York Public Health Institute as a research associate. The move was an important part of developing Kalckar's research biochemical abilities as an enzymologist, because as Paul Berg commented, he “developed a whole new approach to being able to use enzymes in a novel way.” The lab was equipped with a Beckman DU ultraviolet spectrophotometer, introduced in 1941 and still relatively rare; Kalckar rapidly used the instrument to develop several novel enzyme assays. In 1947 he published three papers on purine metabolism enzymes, all of which were highly cited (3,200 citations in 2006). Famous Post-Graduate Student: James D. Watson References External links National Academy of Sciences Biographical Memoir Danish biochemists 1908 births 1991 deaths Washington University in St. Louis fellows University of Copenhagen alumni