SUBSTRATE-SPECIFICITY FOR ISOMERASE ACTIVITY OF MACROPHAGE-MIGRATION INHIBITORY FACTOR AND ITS INHIBITION BY INDOLE-DERIVATIVES

Macrophage migration inhibitory factor (MIF) was discovered as a cytok ine that inhibits random migration of macrophages and concentrates the m at inflammatory loci, We recently reported the tertiary structure of MIF, and revealed its similarity to that of 5-carboxymethyl-2-hydroxy muconate isomerase, Moreover, MIF was found to have isomerase activity converting D-dopachrome, a stereoisomer of naturally-occurring L-dopa chrome, to 5,6-dihydroxyindole-2-carboxylic acid. In this study, are e xamined the effects of a series of compounds analogous to D-dopachrome on the enzyme activity to obtain vital information for identification of a natural substrate of MIF. Adrenochrome, lacking a carboxyl group at position 2 of the indolinequinone ring, could not be a substrate, Several indole-ring-containing compounds with a carboxyl group were in hibitory to D-dopachrome isomerase activity, of which indole-3-acrylic acid was the most potent inhibitor, with an inhibitor constant (K-1) of 2.8 mM. 2,3-Indolinedione, which lacks a complete indole ring or a carboxyl group but has carbonyl groups at positions 2 and 3, apparentl y inhibited the enzyme activity in a competitive or mixed manner with a K-1 of 0.9 mM. Taken together, these facts suggest that the 2-carbox yl group of the substrate is essential for interaction with the active site of MIF.