Comparative biochemistry of urea synthesis. I. Methods for the quantitative assay of urea cycle enzymes in liver.

Elucidation of the various enzymatic reactions of the KrebsHenseleit ornithine-urea cycle (1) stems from several laboratories (l-13), whose primary aims were the isolation, purification, and study of the requirements and mechanism of action of the several enzymes. The present study was undertaken to provide a systematic scheme of analysis for the quantitative determination of the levels of these enzymes in the livers of ureotelic animals. Primarily, the procedures were established to be used in connection with a study of levels of these enzymes in cell-free extracts prepared from the livers of metamorphosing tadpoles (18). The procedures have lent themselves more broadly to comparative studies on the occurrence and levels of these enzymes in mam-