Measurement of half-life of human plasma fibrinogen

fibrinogen half-life (t,,,) was measured in a series of adult volunteers by feeding either [l”N]glycine (loo-350 mg/h) or [ lSN]ammonium chloride (180-350 mg/h) plus sodium benzoate (250-500 mg/h). The lSN enrichment of the hepatic fibrinogen amino acid precursor pool was estimated from the lSN enrich-ment of the urinary hippuric acid. The amount of isotope incorporated into fibrinogen was found by measuring the 15N enrichment of glycine incorporated into fibrinogen. Two types of experiment were run. i) InitiaZ-rate: the subject took the [ 15N]glycine or [ljN]ammonium chloride plus benzoate mix-ture orally in eight equal doses over a 3- to 4-h period. ii) Steady-state: the [ ljN]glycine or [ lSN]ammonium chloride plus benzoate were taken orally in 10 equal doses over a lo- to 12-h period. Giving the isotope at a constant rate results in an approximate isotopic steady state in the body’s free amino acid pools. For both types of experiment a blood and urine specimen were obtained at the end of the 15N feeding period. The results of the experiments were: tliz (days), initial-rate with [l”N]glycine, 3.4 t 1.5 (7), t,,, with [15N]ammonium chloride, 1.4 t 0.6 (7); t1,2 steady-state with [‘“Nlglycine, 2.4 t 0.6 (5), and with [ lSN]ammonium chloride I.6 t 0.4 (5). The numbers in parentheses are the numbers of studies. The values ob-tained with [ljN]glycine were greater than the corresponding [lSN]ammonium chloride values. (P c 0.02 for both methods.) protein synthesis; l”N PLASMA PROTEIN TURNOVER RATES canbe estimatedby measuring either the rate of synthesis or of catabolism