Production of [14C]‐Labeled 3‐Hydroxy‐L‐Kynurenine in a Butterfly, Heliconius charitonia L. (Heliconidae), and Precursor Studies in Butterfly Wing Ommatins

A method was developed to produce radiolabeled 3-hydroxy-L-kynurenine by injection of [14C]-L-tryptophan into pupae of the heliconid butterfly, Heliconius charitonia, which was converted into [14C]-3-hydroxy-L-kynurenine and deposited as a wing pigment. Extractions of 3-hydroxykynurenine (3-OHK) with 60% methanol from wings yielded in 14.4 micrograms per mg dry weight. In extracts from yellow wing areas, 3-OHK represented 100% of detectable amino acids. Resulting specific radioactivity of [14C]-3-OHK was between 0.05 and 0.07 mCi/mmol when 0.5 microCi [14C]-tryptophan was injected into pupae 1 or 2 days before emergence of the butterfly. Incorporation of [14C]-3-OHK into wing ommochromes was studied in nymphalid butterflies, Araschnia levana and Precis coenia. After injection into pupae [14C]-3-OHK as well as [14C]-tryptophan were specifically incorporated into red and red-brown wing scales as shown by autoradiography. The same incorporation occurred in isolated wings after incubation in Grace's medium containing [14C]-3-OHK. In Araschnia levana, [14C]-3-OHK offered to left wing pairs was incorporated into dihydroxanthommatin six times more effectively than [14C]-tryptophan offered to right wing pairs from the same specimen. Therefore, 3-OHK seems to be the ultimate precursor of wing ommatins.