Measurement of hemoglobin synthesis rate in vivo using a stable isotope method

We developed a method to measure hemoglobin synthesis rate (SynHb) in human s, assuming that free glycine in the red blood cell (RBC) represents free g lycine in bone marrow for hemoglobin synthesis. The present rat study exami nes this assumption of the method and quantifies SynHb in rats. Sprague-Daw ley rats (n = 9) were studied, [2-C-13]glycine was intravenously infused ov er 24 h (2.5 mg kg(-1) h(-1)), blood was drawn for glycine and heme isolati on, and bane marrow was harvested for glycine isolation. Isotopic enrichmen ts of glycine and heme were measured, fractional hemoglobin synthesis rate (fSynHb% day(-1)) was calculated, and from this a value for SynHb (mg g(-1) day(-1)) was derived. Mean body weight was 446 +/- 10 g (mean +/- SE) and hemoglobin concentration was 14 +/- 0.5 g dl(-1). At 24 h, the mean isotopi c enrichment, atom percentage excess (APE), of the RBC free glycine (1.56 /- 0.18 APE) was similar to the bone marrow (1.68 +/- 0.15 APE). The rate o f incorporation of C-13 into heme increased over time from 0.0004 APE/h bet ween 6 and 12 h, to 0.0014 APE/h between 12 and 18 h, and 0.0024 APE/h betw een 18 and 24 h, Consequently, fSynHb (1.19 +/- 0.32, 2.92 +/- 0.66, and 4. 22 +/- 0.56% day(-1), respectively) and SynHb (0.11 +/- 0.03, 0.28 +/- 0.05 , and 0.42 +/- 0.05 mg g(-1) day(-1), respectively) showed similar patterns over the 24-h study period. We conclude that (1) enrichment of free glycin e in the circulating RBC approximates enrichment of bone marrow free glycin e for heme formation and (2) this pattern of hemoglobin synthesis rate is r eflecting the characteristic release and gradual maturation of reticulocyte s in the circulation. (C) 2001 Academic Press.