We have identified eIF5A as the only cellular protein that contains an unusual amino acid, hypusine [Nepsilon- (4-amino-2-hydroxybutyl)lysine], and have established that hypusine biosynthesis occurs posttranslationally by two sequential enzymatic reactions. In the first step deoxyhypusine synthase catalyzes the transfer of the butylamine moiety of the polyamine spermidine to a specific lysine residue in the eIF-5A precursor protein to form an intermediate, deoxyhypusine residue. In the latter step, this intermediate is converted to hypusine by a metalloenzyme deoxyhypusine hydroxylase. Hypusine is essential for the activity of eIF-5A and for eukaryotic cell proliferation. Thus hypusine biosynthetic steps present novel targets for intervention in eukaryotic cell proliferation. In addition to the X-ray crystal structure of human deoxyhypusine synthase in a complex with NAD, a new structure for a ternary complex between the enzyme, NAD and the inhibitor, GC7, has been determined. These structures reveal NAD binding sites and an active site pocket where spermidine is presumed to bind. The role of a number of amino acids predicted to be involved in the binding of NAD, of spermidine, and those critical for the catalysis, was assessed by site-directed mutagenesis. Molecular modeling of the spermidine binding site should aid development of specific inhibitors of deoxyhypusine synthase that may be useful as anti-proliferative agents. We have tested the effects of inhibitors of deoxyhypusine hydroxylase on human vein endothelial cell (HUVEC) proliferation and angiogenesis. These compounds inhibited deoxyhypusine hydroxylase and proline hydroxylase, and caused cell cycle arrest in G1. Of the five metal chelating inhibitors i.e. mimosine, 2,2?-dipyridyl, deferiprone, deferoxamine and ciclopirox, the antifungal drug ciclopirox was the most effective in the inhibition of the two protein hydroxylases, HUVEC proliferation and angiogenesis in two model assays. Furthermore, this compound exerts strong antiproliferative effects on a panel of human cancer cell lines. These findings suggest that ciclopirox is a valuable candidate for clinical trials in the treatment of solid tumors.
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