It was shown that N-ethylmaleimide (NEM) reacts rapidly and specifically with sulfhydryl groups to exert antimitotic action (6), and that this chemical nature of the substance is utilized for the titration of such groups in proteins (12). Thereafter, many studies on the bactericidal and fungicidal actions of NEM and its derivatives were reported (7,10).In the present study, we synthesized N-substituted maleimides including Nalkyl, N-aralkyl, and N-1,3,4-thiadiazoly1 derivatives by the method of Kanaoka et al (9) and examined the antifungal activity of these compounds. The crystallized compounds to be tested were purified by recrystallization from the solvent and other liquid chemicals by silica gel chromatography using a mixture of ethyl acetate and chloroform as the eluent. The purity was verified by thin-layer chromatography, elementary analysis, melting point (mp) and mass spectrometry. The data for these chemicals are listed in Table 1.All the test chemicals were dissolved in dimethyl sulfoxide (DMSO) and were subjected to the following assay. By the plate dilution method, all the chemicals were tested for their inhibitory activities against Candida albicans (a clinical isolate), Aspergillus niger (IFO 6341), Penicillium citrinum (IFO 6352), and Trichophyton mentagrophytes (a clinical isolate) in Sabouraud dextrose agar (Difco) at pH 5.6 and 7.0 in the absence and presence of 10% beef serum. DMSO alone had no antifungal activity on the fungal cells under the experimental conditions used in this study. We used amphotericin B (Sankyo Co., Ltd., Tokyo, Japan) as a reference compound, which is clinically useful for the treatment of fungal diseases. Table 2 shows the antifungal activity of the N-substituted maleimide derivatives. N-alkyl maleimide derivatives were inactive against the tested microorganisms under the present conditions. On the other hand, N-thiadiazolyl derivatives of maleimide (TDZ, 5-Me-TDZ, and 5-Et-TDZ) exhibited antifungal activity against T. mentagrophytes at relatively low concentrations compared with N-alkyl maleimide derivatives, but they were less active than amphotericin B.Studies on interaction of N-alkylmaleimides with the sulfhydryl groups of several enzymes have been reported (1,2,5,8). They concluded that the Nalkylmaleimides interact with a hydrophobic region in the enzyme based on the facts that the inactivation of sulfhydryl groups is greatly affected by the chain length of the N-alkyl substituents of the maleimide derivatives, and that hydrophobicity 1237
