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It is possible that antifungal drugs with novel modes of action will provide favorable options to treat fungal infections. In the course of our screening for antifungal compounds acting on the cell wall, a pyridobenzimidazole derivative with unique activities, named D75-4590, was discovered. During treatment of Saccharomyces cerevisiae with D75-4590, (i) incorporation of [ 14 C]glucose into the ␤-1,6-glucan component was selectively reduced, (ii) proteins released from the cell had lost the ␤-1,6-glucan moiety, and (iii) cells tended to clump, resulting in impaired cell growth. Genetic analysis of a D75-4590-resistant mutant of S. cerevisiae indicated that its primary target was Kre6p, which is considered to be one of the ␤-1,6-glucan synthases. These results strongly suggest that D75-4590 is a specific inhibitor of ␤-1,6-glucan synthesis. D75-4590 showed potent activities against various Candida species. It inhibited hyphal elongation of C. albicans as well. KRE6 is conserved in various fungi, but no homologue has been found in mammalian cells. These lines of evidence indicate that D75-4590 is a promising lead compound for novel antifungal drugs. To our knowledge, this is the first report of a ␤-1,6-glucan inhibitor.

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In mammals, histamine is inactivated principally by two enzymes: histamine N-methyltransferase (HMT; EC 2.1.1.8) and diamine oxidase (DAO; EC 1.4.3.6.). The cDNA clone of human HMT (hHMT) has been isolated from a cDNA library of human kidney and its nucleotide, and deduced amino acid sequences have been determined. One clone, phHMT-1, containing an insert of 1.4 kb, was confirmed to encode HMT by transient expression of HMT activity in COS cells. hHMT consists of 292 amino acid residues [relative molecular weight (M(r)) = 33,279] and shares 82% identity with that of rat HMT. Northern blot analysis with hHMT cDNA probe revealed that 1.6-kb HMT mRNA transcript was expressed in the lung, nasal polyps, and kidney. HMT activity was measured in human trachea and bronchi. In addition, the contractile response of isolated human bronchi to histamine was potentiated in the presence of an HMT inhibitor, SKF 91488, but a DAO inhibitor, aminoguanidine, was without effect. These results suggest that HMT plays an important role in degrading histamine and in regulating the airway response to histamine. Therefore, the level of HMT gene expression in human airway may be one of the critical factors determining the airway responsiveness to histamine. In situ chromosomal hybridization demonstrated that human HMT gene was localized in chromosome 1 p32.

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