Figure 2. PTC carbamylates ornithine in addition to putrescine.

(A) Approach to equilibrium in the carbamylation of ornithine (open symbols) using either OTC (squares) or PTC (triangles) from E. faecalis as catalyst and comparison with the equilibrium for putrescine carbamylation catalyzed by PTC (closed symbols). Tubes containing the indicated amounts of either OTC or PTC in 0.25 ml of 0.1 M Tris-HCl pH 8.5, 0.4 mg/ml bovine serum albumin, 10 mM carbamylphosphate, and 10 mM of either ornithine or putrescine (as indicated), were incubated 10 min at 37°C. Then 0.1 ml of cold 20% (w/v) trichloroacetic acid was added, and the amount of citrulline or carbamylputrescine, respectively, was determined [9]. The results show the amount of these ureido compounds in the 0.25-ml incubation mixtures. (B) Inhibition by increasing concentrations of PAPU of the transcarbamylase activities of E. faecalis PTC using putrescine (closed circles) or ornithine (open triangles) as substrates, and lack of inhibition of E. faecalis OTC (open squares). Activities are given as a percentage of the activities in the absence of PAPU. A single curve has been fitted to the results observed for PTC activity with both putrescine and ornithine as substrates.