supplied. Cohen and Grisolia (1950) concluded that in the synthesis of citrulline from ornithine, carbamyl-L-glutamic acid was an intermediate; it was not active in the absence of ammonium ion. The Cohen group and the Henry A. Lardy group (Patricia MacLeod, Grisolia, Cohen, and Lardy) (1949) joined forces to investigate the role of biotin in the synthesis of citrulline from ornithine. Biotin-deficient rat livers were only about half as active as livers from pair-fed controls. Injection of biotin twenty-four hours before testing the rats restored their livers to normal activity.

Earlier Cohen had questioned the conclusion of Braunstein and Kritzman that transamination was a general reaction among the amino acids, as he had found much higher activities among glutamic acid, aspartic acid, and alanine. P. S. Cammarata and Cohen (1950) reexamined the issue and reversed their opinion on the generality of transamination when they found that twenty-two amino acids in addition to alanine and aspartic and glutamic acids transaminated with aqueous extracts of pig heart, liver, and kidney. They stated, "Each transamination reaction appears to be due to a different transaminase." They had devised new methods for these tests and had found that pyridoxal phosphate accelerated the reactions.

Phil Cohen rather promptly established himself as an expert in the area of nitrogen metabolism in animal tissues, and in 1945 he contributed a chapter on proteins and amino acids to the Annual Review of Biochemistry . This review had very broad coverage of the topics of protein synthesis, plasma and tissue proteins, amino acid requirements, intermediary metabolism, deamination, transamination (a rather short treatment considering this was an area of special expertise for Cohen), and about a dozen other topics. Later he contributed a wider ranging review with Henry J. Sallach (1961) in a book entitled Metabolic Pathways. As they pointed out,