PRECURSOR POOL FOR HEPATIC PROTEIN-SYNTHESIS IN HUMANS - EFFECTS OF TRACER ROUTE INFUSION AND DIETARY PROTEINS

The estimation of the hepatic protein synthesis precursor pool was inv estigated through the measurement of very low-density lipoprotein (VLD L) apolipoprotein (apo)B100 labeling in healthy volunteers. L-[1-C-13] leucine and L-[5,5,5-H-2(3)] leucine were administered intravenously a nd intragastrically, respectively. Subjects were continuously fed with isoenergetic meals providing either 16% protein or no protein. The la beling of leucine incorporated into VLDL apoB100 (leucine-apoB) was lo wer than plasma leucine or alpha-ketoisocaproate (KIC) enrichments wit h the intravenous tracer. By contrast, with the oral tracer, leucine a poB enrichment was higher than either plasma free leucine or KIC label ing. The KIC and leucine-apoB enrichments relative to plasma leucine e nrichment were not affected by protein intake. Albumin or fibrinogen s ynthesis rates were similar whatever the administration route of the t racer when leucine-apoB was used to indicate the precursor, which was not the case for plasma leucine or KIC. The present data suggest that leucine-apoB enrichment represents a reliable indicator of th hepatic precursor pool for protein synthesis. The effect of dietary protein on the calculated rates of albumin and fibrinogen synthesis is also repo rted in relation to the choice of the precursor.