Comparison of serine and hippurate as precursor equivalents during infusion of [ 15N]glycine for measurement of fractional synthetic rates of apolipoprotein B of very-low-density lipoprotein

Abstract

Enrichment in hippurate has been measured to indicate precursor enrichment during glycine tracer infusion studies to estimate fractional synthetic rates of individual hepatic export proteins. However, hippurate tends to overestimate precursor enrichment. Since glycine is rapidly converted to serine by liver cells, we compared tracer enrichment in hippurate and serine with that of glycine incorporated into apolipoprotein (apo) B-100. Ten healthy control subjects were studied in the postabsorptive state during an 8-hour primed-constant infusion of [ 15 N]glycine (10 μmol · kg −1 · h −1). Apo B of very-low-density lipoprotein (VLDL) was isolated by standard ultracentrifugation and isopropanol precipitation. Glycine and serine were isolated from plasma and hydrolyzed apo B, hippurate was isolated from plasma, and [ 15 N]enrichment was determined by gas chromatography-mass spectrometry. Enrichment in serine and glycine isolated from apo B was identical at all time points, and their enrichment in apo B increased asymptotically, approaching an apparent plateau (mean ± SD: 91% ± 10% of calculated plateau at 8 hours) that was taken to represent hepatic protein precursor enrichment. Enrichment in both plasma serine and hippurate followed a biphasic pattern and continued to increase until the end of the study, raising the possibility that precursor enrichment had not reached a steady state during the study. The apo B plateau was lower (factor 0.76 ± 0.27) than the final enrichment in hippurate and higher (factor 1.38 ± 0.36) than that in plasma serine; however, predictions of protein precursor enrichment based on either metabolite were flawed by a large coefficient of variation (35% v 26%). We conclude that glycine enrichment in the hepatic protein precursor pool may not be constant during an 8-hour infusion study, and that only a rough approximation of this level may be obtained using either enrichment in plasma hippurate or plasma serine.