ESTIMATES OF THE EFFECT OF FEEDING ON WHOLE-BODY PROTEIN-DEGRADATION IN WOMEN VARY WITH THE AMINO-ACID USED AS TRACER

We measured how feeding status affects the kinetics of multiple indisp ensable amino acids in four adult female subjects studied both in fed and fasted state. The subjects ingested one dose of uniformly C-13-lab eled algae (Spirulina platensis). The isotopic enrichments (measured w ith negative chemical ionization gas chromatography-mass spectrometry) of the branched chain amino acids, phenylalanine, lysine and threonin e were followed for 24 h in both the plasma and in VLDL-apolipoprotein B-100 (apoB-100). Easting lowered body protein degradation when measu red with the branched chain amino acids, increased it when measured wi th phenylalanine and had no statistically significant effect when dete rmined from the kinetics of lysine and threonine. These apparent diffe rences challenge the adequacy of current models of whole-body protein turnover. The ratio of the peak labeling of amino acids in plasma and apoB-100 was used as an estimate of the isotopic dilution in the hepat ic pool. In contrast to our earlier observations during intravenous tr acer amino acid administration, in the present study fasting lowered t he ratio of the peak isotopic enrichments of apoB-100 and plasma amino acids. This supports our contention that feeding increases the use of hepatic portal amino acids for hepatic secretory protein synthesis.