Inhibitory Effects of Flavonolignans from Silybum marianum (L.) Gaertn (Milk Thistle) on Function of Aldehyde Oxidase and Xanthine Oxidase in Rats

Abstract

Background: Silybum marianum (L.) Gaertn (Milk thistle) is a medicinal herb fraught with flavonolignans known as silymarin. Methods: We investigated and compared in vitro effects of commercial silibinin (mixture of two diastereomers, silybin A and silybin B) and milk thistle flavonolignans on the kinetic parameters and activities of aldehyde oxidase (AO) and xanthine oxidase (XO) of rat liver. Samples of milk thistle seeds were collected from different locations in Iran and their silymarin extract (contained silibinin flavonolignans) were extracted and quantified. Partially purified hepatic AO and XO were prepared from liver homogenates of rats. AO and XO activities were spectrophotometrically determined in the presence of silibinin (10, 50 and 100 mu M) and silymarin extracts (10, 50 and 100 mu g/ml) using phenanthridine and xanthine as substrates, respectively. Silymarin extract (100 mu g/ml) and silibinin (100 mu M) were able to inhibit the rat hepatic XO activities by 65 +/- 4 and 70 +/- 1%, respectively. Furthermore, the AO activity was reduced to 62 perpendicular to 1 and 70 perpendicular to 3% in the presence of silymarin extracts and silibinin, respectively. Results and Conclusion: Kinetic studies indicated that AO is inhibited by silibinin in an uncompetitive manner, whereas this flavonolignan was found as a mixed inhibitor of the XO activity. However, compared to the menadione (an AO inhibitor) and allopurinol (the XO inhibitor), silibinin and silymarin extracts had a lower inhibitory effect.