Utilization of Glycine Nitrogen at Various Levels of Glycine Intake

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Abstract
Metabolism of approximately 20 mg of N15 from glycine, ingested as a single dose, was studied in each of 2 dogs, in 5 successive experiments wherein total intake of free glycine increased stepwise from 1.5 to 19.5 g/day. Output of N15 as total nitrogen, urea, and ammonia, as well as the amount in fibrinogen, were determined 6, 12, 24, and 48 hours after ingestion. Percentage of N15 retained after 24 hours, and the percentage observed in fibrinogen, were both inversely proportional to the logarithms of total free glycine intake. The 13-fold increase in glycine intake reduced the former percentage by about one third, and the latter by about two thirds. Rate of urea formation, expressed as milligrams of urea nitrogen per kilogram of body weight per hour, was a linear function of glycine intake during the most active period of catabolism (second 6-hour period after ingestion). Percentage of urinary ammonia originating from ingested glycine during the first two 6-hour periods increased linearly with glycine intake during the first 4 levels of supplementation, but not at the highest level. Glycine evidently had a sparing effect on formation of ammonia from other sources, but this effect was limited.