COOPERATION OF DOPACHROME CONVERSION FACTOR WITH PHENOLOXIDASE IN THEEUMELANIN PATHWAY IN HEMOLYMPH OF LOCUSTA-MIGRATORIA (INSECTA)

Dopachrome Conversion Factor (DCF) was found in the plasma of the locu st Locusta migratoria. It has an apparent molecular mass of 85,000. It s K-m was 0.2 mM at 22 degrees C and pH 7 with L-dopachrome as substra te. It had a high substrate specificity for L-dopachrome, methyl-L-dop achrome and L-dopachrome methyl ester but no activity on the correspon ding D-isomers or on dopaminechrome. DCF was devoid of any phenoloxida se activity. Under action of DCF, L-dopachrome was converted into dihy droxyindole, which showed that a decarboxylation occured in the course of reaction. Locust DCF was inhibited by indole-3-propionic acid but not by indole-3- or indole-2-carboxylic acid. It was also inhibited by L-tryptophan in a competitive manner. Inhibition and substrate specif icity suggest that a carboxyl group, either free or as a methyl ester, is necessary but not sufficient for enzyme recognition. When purified prophenoloxidase was activated and then added to dihydroxyindole eith er prepared by chemical synthesis or obtained by action of purified DC F on dopachrome, black pigments with a maximum absorption at 540 nm we re generated. Therefore in the eumelanin pathway of locust plasma, phe noloxidase can catalyze the reaction that converts the product generat ed by DCF. (C) 1998 Elsevier Science Ltd. All rights reserved.