Coelenterazine is an imidazopyrazinone compound (3,7-dihydroimidazopyrazin-3-one structure) that is widely distributed in marine organisms and used as a luciferin for various bioluminescence reactions. We have used electrospray ionization-ion trap-mass spectrometry to investigate whether the deep-sea luminous copepod Metridia pacifica is able to synthesize coelenterazine. By feeding experiments using deuterium labeled amino acids of L-tyrosine and L-phenylalanine, we have shown that coelenterazine can be synthesized from two molecules of L-tyrosine and one molecule of L-phenylalanine in M. pacifica. This is the first demonstration that coelenterazine is biosynthesized from free L-amino acids in a marine organism. (C) 2009 Elsevier Inc. All rights reserved.