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.