Cherqui A, Duvic B, Reibel C, Brehélin M
Laboratoire de Pathologie Comparée, INRA-CNRS (URA 2209), Montpellier, France.
Insect Biochem Mol Biol. 1998 Nov;28(11):839-48. doi: 10.1016/s0965-1748(98)00076-9.
Dopachrome Conversion Factor (DCF) was found in the plasma of the locust Locusta migratoria. It has an apparent molecular mass of 85,000. Its K(m) was 0.2 mM at 22 degrees C and pH 7 with L-dopachrome as substrate. It had a high substrate specificity for L-dopachrome, methyl-L-dopachrome and L-dopachrome methyl ester but no activity on the corresponding D-isomers or on dopaminechrome. DCF was devoid of any phenoloxidase activity. Under action of DCF, L-dopachrome was converted into dihydroxyindole, 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 specificity 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 either prepared by chemical synthesis or obtained by action of purified DCF on dopachrome, black pigments with a maximum absorption at 540 nm were generated. Therefore in the eumelanin pathway of locust plasma, phenoloxidase can catalyze the reaction that converts the product generated by DCF.
在飞蝗的血浆中发现了多巴色素转换因子（DCF）。其表观分子量为85,000。在22℃和pH 7条件下，以L - 多巴色素为底物时，其米氏常数（K(m)）为0.2 mM。它对L - 多巴色素、甲基 - L - 多巴色素和L - 多巴色素甲酯具有较高的底物特异性，但对相应的D - 异构体或多巴胺色素没有活性。DCF没有任何酚氧化酶活性。在DCF的作用下，L - 多巴色素转化为二羟基吲哚，这表明反应过程中发生了脱羧反应。飞蝗DCF受到吲哚 - 3 - 丙酸的抑制，但不受吲哚 - 3 - 羧酸或吲哚 - 2 - 羧酸的抑制。它也受到L - 色氨酸的竞争性抑制。抑制作用和底物特异性表明，游离的或作为甲酯的羧基对于酶的识别是必要的，但不是充分的。当纯化的前酚氧化酶被激活，然后添加到通过化学合成制备或通过纯化的DCF对多巴色素的作用获得的二羟基吲哚中时，会产生在540 nm处具有最大吸收的黑色色素。因此，在飞蝗血浆的真黑素途径中，酚氧化酶可以催化将DCF产生的产物转化的反应。
