Inhibition characteristics of hypericin on rat small intestine glutathione-S-transferases

Glutathione-S-transferases constitute a family of enzymes involving in the detoxification of xenobiotics, signalling cascades and serving as ligandins or/and catalyzing the conjugation of various chemicals and drugs. The widely expressed cytosolic GST-π is a marker protein in various cancers and its increased concentration is linked to drug resistance. GST-π is autoregulated by S-glutathionylation and it catalyzes the S-glutathionylation of other proteins in response to oxidative or nitrosative stress. S-glutathionylation of GST-π results in multimer formation and the breakage of ligand binding interactions with c-Jun NH2-terminal kinase (JNK). Another widely expressed GST enzyme, GST-α is assumed as a marker in hepatocellular damage, is implicated in cancer, asthma, cardiovascular disease and response to chemotherapy.Although, it was shown that hypericin binds and inhibits GST-α and GST-π, the inhibition characteristics have not been investigated in detail. The aim of this study was to investigate the effects of hypericin on major GSTs; GST-α and GST-π purified from rat small intestine. When GSH used as varied substrate the inhibition pattern with hypericin was uncompetitive for GST-α (Ki = 0.16 ± 0.02 μM) and noncompetitive for GST-π (Ki = 2.46 ± 0.43 μM). While using CDNB (1-chloro-2,4-dinitrobenzene) as the varied substrate, the inhibition patterns were noncompetitive for GST-α and competitive for GST-π; Ki values for GST-α and GST-π were 1.91 ± 0.21 and 0.55 ± 0.07 μM, respectively. Since hypericin accumulated in cancer cells and important in photodynamic therapy (PDT), inhibition of GST-α and GST-π by hypericin might increase the effectivity of the treatment. Considering that GST-π is responsible for the drug resistance its inhibition might increase the benefit obtained from chemotherapy.