Ornithine-catalyzed urea formation in liver homogenate

Abstract

Heretofore attempts to obtain synthesis of urea from ammonia (and carbon dioxide) in cell-free extracts have been unsuccessful. We have found the reaction to proceed in guinea pig liver homogenate. The following is the reaction mixture which has given the highest yields obtained so far: L-ornithine (0.00075 M), ammonia (0.0025 M), L-glutamate (0.01 M), oxalacetate (0.005 M), ATP (0.00025 M), and 0.33 gm. of homogenized liver in a final volume of 3.5 ml. The following are typical increases in urea over the blank observed in 1 hour, in micrograms: ornithine + ammonia 0, glutamate + oxalacetate 0, glutamate + oxalacetate + ornithine 49, glutamate + oxalacetate + ammonia 105, glutamate + oxalacetate + ornithine + ammonia 317, glutamate + ocalacetate + ornithine + ammonia (without ATP) 150.