they were concerned primarily with enzymatic systems involved in the transformation or transfer of the amino, amine and amide nitrogen moiety of amino acids, amines, and amides, and they gave preference to studies that emphasized the enzymatic aspects of chemical transformations. With this definition of their interests they then covered the field rather thoroughly in a 78-page treatment supported by 511 references. They even brought the subject up to date with an addendum.

A good summary of his extensive work on the changes of nitrogen metabolism during metamorphosis of tadpoles to frogs is given by George W. Brown, Jr., and Cohen in a symposium presentation (1958). By 1958 Cohen's group had studied this intriguing transformation of the ammonia-excreting tadpole to the urea-excreting frog rather extensively. They had concluded that the Krebs-Henseleit ornithineurea cycle was induced during metamorphosis. They pointed out that the nitrogen metabolism observed involved a highly integrated system consisting of nine enzymes operating in three subcycles. The net reaction at steady state kinetics converted two moles of ammonia and one mole of bicarbonate to urea at the expense of three moles of ATP. The investigators used tadpoles of the giant bullfrog, which often have livers that weigh over a gram. As the ratio of the hind limb/tail increased, the percentage of nitrogen recovered as urea increased dramatically. Brown and Cohen followed changes not only in the ammonia to urea ratio with development but the changes in a variety of other pertinent enzymes as well. They present an interesting discussion of the evolutionary development of the urea cycle.

Phil Cohen was a graduate student at Wisconsin during the 1930s and did his postdoctoral stints with Krebs in England and then at Yale. When he returned to Wisconsin it was an era of great research activity on respiratory enzymes.