Purpurin as a Specific Inhibitor of Spermidine-Induced Autoactivation of the Protease Plasma Hyaluronan-Binding Protein

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Plasma hyaluronan-binding protein (PHBP), a serine protease that can activate coagulation factor VII and prourokinase, circulates as a single-chain form (pro-PHBP), and is autoproteolytically converted to an active two-chain form with the aid of an effector such as spermidine and heparin. In this study, we screened natural sources for inhibitors of spermidine-induced pro-PHBP autoactivation. As an active agent, we purified bikaverin from a culture of a fungus. Bikaverin inhibited spermidine-induced autoactivation with an IC50 of 0.45 μM, while it also inhibited the active form of PHBP (IC50=0.8 μM). Additional screening of related compounds led to the identification of purpurin, a plant anthraquinone, as a specific inhibitor: IC50=6.6 μM for spermidine-induced autoactivation; no inhibition of heparin-induced autoactivation and active PHBP. Alizarin and emodin, which structurally differed from purpurin in the position or the number of the hydroxyl groups, were less active and nonspecific. Thus, the position and/or the number of the hydroxyl group affect both the potency and selectivity of the anthraquinone inhibitors.