Isolation and Quantification of L-Tryptophan from Protaetia brevitarsis seulensis Larvae as a Marker for the Quality Control of an Edible Insect Extract

<i>Protaetia brevitarsis seulensis</i> (Kolbe, 1886) larvae have traditionally been used in East Asian medicine and have recently attracted attention as functional food ingredients because of their pharmacological potential. However, chemical investigations remain limited, and no marker compounds have been established for quality control. This study aimed to isolate and identify a primary constituent from the 70% ethanol extract of <i>P. brevitarsis</i> (PBE) and to develop an analytical method for its quantification. Among the solvent-partitioned fractions, the <i>n</i>-butanol fraction (PBE-B) exhibited a major peak in HPLC analysis. The compound was purified through a combination of vacuum liquid chromatography (VLC), medium-pressure liquid chromatography (MPLC), and recycling preparative HPLC. Its structure was identified as L-tryptophan based on HR-ESI-MS and NMR spectroscopy. Quantitative analysis was conducted using HPLC-DAD under optimized analytical conditions, employing a Thermo Scientific™ Acclaim™ Polar Advantage II column and an acidified mobile phase (0.1% formic acid in water and methanol) to improve resolution. The method demonstrated excellent linearity (r² > 0.9999), and the L-tryptophan content in PBE was determined to be 1.93 ± 0.05 μg/mg. The analyte was well separated with minimal interference, supporting the reproducibility of the method. These results indicate that L-tryptophan is a promising candidate Q-marker for the quality control of <i>P. brevitarsis</i> extracts.