Plasma concentrations of p- and m-hydroxyphenylacetic acid and phenylacetic acid in humans: gas chromatographic--high-resolution mass spectrometric analysis.

Conjugated and unconjugated phenylacetic acid and m- and p-hydroxyphenylacetic acid have been determined in the plasma of normal, healthy subjects after fasting, consumption of a meal and ingestion of deuterium labelled amine precursors, by high-resolution gas chromatography--high resolution mass spectrometry with selected ion monitoring of their trifluoroethyl-pentafluoropropionyl derivatives. We observed tht all three conjugated acids are higher in fasting than in non-fasting subjects, and unconjugated phenylacetic acid was lower. Ingestion of deuterium-labelled amine precursors resulted in the appearance in the blood of the correspondingly labelled acids, a peak in the concentrations being reached about 1 h after consumption. Conjugated and unconjugated acids as expected increased following the consumption of a meal. Unconjugated phenylacetic acid was significantly higher in females than in males. Most values tended to increase with age, with male unconjugated and conjugated m-hydroxyphenylacetic acid and female conjugated phenylacetic and m-hydroxyphenylacetic acids increasing significantly.