Our objective was to develop a stable isotopic method to measure the synthesis rates of retinol-binding protein (RBP) and transthyretin (TTR). Both proteins were isolated from human and pig plasma by sequential immunoprecipitation and purified by SDS-polyacrylamide gel electrophoresis under denaturing conditions. Both human and pig anti-RBP precipitates contained a peptide (TTR2) that had a molecular mass that was similar but not identical to that of TTR subunit. The N-terminal amino acid sequence of porcine TTR2 was highly but not completely homologous with porcine TTR. Human TTR2 showed no homology with TTR but was completely homologous with an internal sequence of human fibrinogen alpha chain. To measure the fractional rates of synthesis (FRS) of these peptides, six infant pigs were infused with [2H3]leucine at a constant rate for 6 h, and the amount of [2H3]leucine incorporated into the proteins was measured by negative chemical ionization gas chromatography-mass spectrometry. The plateau isotope ratio of plasma very low density lipoprotein apoB-100-bound leucine was used to estimate the isotopic enrichment of hepatic protein synthetic precursor pool. The mean FRS (% h +/- S.E.) of TTR (1.97 +/- 0.13) and RBP (3.89 +/- 0.07) were significantly different. The FRS of TTR2 was low (0.31 +/- 0.19) relative to that of RBP and TTR. Thus, three different peptides with different turnover rates seem to be involved in the transport of retinol.