Table 2.

Isotopic enrichment of urea and creatine in hepatocyte incubations with ¹⁵N-ammonium chloride

	Urea (M + 1)	Urea (M + 2)	Creatine (M + 1)	Creatine (M + 2)
−Ornithine	43.0±10.6	55.3±6.1	2.9±3.5	4.1±4.7
+Ornithine	38.3±7.9	61.3±6.0	1.9±1.8	5.9±5.9

Values are expressed as atom percent excess as means ± SD; n = 4 hepatocyte preparations. Incubations included 5 mg dry mass hepatocytes for 30 min at 37°C. Hepatocytes were incubated with (in mM) 1 ¹⁵N-ammonium chloride, 2 pyruvate, 0.5 methionine, 1 glycine, and 1 ornithine where indicated. M, mass isotopomers of molecular weight 60 g/mol for urea and 130 g/mol for creatine. M + 1 and M + 2 are, respectively, 1 and 2 mass units heavier than M.