Isolation of Allosteric Tryptase Inhibitor from Methanol Extract of Rhubarb and Enhancement of Its Tryptase Inhibitory Activity by Compounds That Were Screened by In Silico Screening

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Tryptase, which is abundant in human mast cells and is involved in allergic inflammations such as asthma, is a serine protease. We isolated a tryptase inhibitor, procyanidin B8 3,3′-di-O-gallate (PB8GG’), a tannin, from the methanol extract of rhubarb (RHEI RHIDOMA), which is a traditional Chinese medicine (Kampo medicine in Japan). Since it did not inhibit another serine protease trypsin, PB8GG’ specifically inhibited tryptase. A standard kinetic analysis of the inhibitory fashion of PB8GG’ against tryptase suggested that PB8GG’ inhibited tryptase in an allosteric manner. We searched for other tannins like PB8GG’ expected to bind tryptase using AutoDock vina. Two ellagitannins, carpinins B and E, isolated from young leaves of Carpinus japonica were selected as candidates of tryptase inhibitors. Carpinins B and E themselves had almost no inhibitory activity against tryptase but enhanced the inhibitory activity of PB8GG’ against tryptase. This is an example that shows that a combination of an allosteric inhibitor with other compounds that bind but have no inhibitory activity can be used to develop a clinically useful combinatorial enzyme inhibitor.