The course and magnitude of spontaneous increase in ammonia concentration in plasma on standing were investigated with EDTA-treated blood specimens from 36 healthy subjects with use of a sensitive and precise enzymic method. Over 90 min, the rates of increase were virtually constant at fixed temperature. The mean (and SE) rates at 0, 20, and 37 degrees C were 3.9 (0.23), 5.2 (0.23), and 25.2 (0.59) mumol/L per hour, respectively. At these temperatures, the plasma contributed at most 7%, 15%, and 10%, respectively, to the formation of ammonia in whole blood. In view of the medical needs and the measured rates of ammonia increase, an interval of 15 min between blood sampling and the start of centrifugation may be tolerated at a specimen temperature of 0 degree C. Rates of ammonia increase showed significant correlations with erythrocyte and platelet count as well as with the plasma activities of gamma-glutamyltransferase (EC 2.3.2.2) and alanine aminotransferase (EC 2.6.1.2).

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Owing to increased enzymatic hydrolysis of glutamine, additional ammonia is formed in blood and plasma specimens with increased gamma-glutamyltransferase activity (gamma-GT, EC 2.3.2.2.). The close correlation between gamma-GT and glutaminase (EC 3.5.1.2) activities (r = 0.97) in plasma, the inhibition with 6-diazo-5-oxo-L-norleucin or borate plus serine, and the activation with maleate clearly show that gamma-GT itself is the glutamine-deamidating enzyme in plasma. Under pathological conditions, increased gamma-GT activity can increase the rate of ammonia formation in plasma more than 30-fold the mean values of healthy subjects. Increased in vitro formation of ammonia caused by increased gamma-GT activity is an important source of false-positive findings in ammonia determination. Because of the high prevalence of pathological gamma-GT activities in clinical populations, blood specimens for ammonia analysis should be preserved by addition of borate plus serine.

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