Measurement of albumin synthesis in humans: a new approach employing stable isotopes.

A new method for measuring albumin synthesis in humans with stable isotopes is presented. This can readily be applied in most clinical conditions, even when albumin losses are occurring or when repeated assessment is required. After rapid intravenous injection of a large dose of [13C]leucine (57 mg/kg body wt, 19.4 atoms%), plasma samples were taken at intervals up to 90 min. The enrichment of free leucine in plasma measured by gas chromatography-mass spectrometry rose to a peak at 10 min and then fell slowly, whereas that in liver biopsies (from surgical patients) ranged from 101.5 to 80.5% of the plasma value between 10 and 90 min after injection. The fractional synthesis rate (FSR) was calculated by dividing the increase in enrichment of leucine in albumin, measured by gas isotope ratio mass spectrometry, by the area under the plasma free leucine enrichment vs. time curve after allowing for the period between synthesis of the protein and its secretion into the plasma. The FSR in healthy postabsorptive males was 7.2 +/- 1.3%/day, and the absolute synthesis rate was 157 +/- 39 mg.kg body wt-1.day-1. These rates are comparable to those obtained by other methods.