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 µM) and silymarin extracts (10, 50 and 100
μg/ml) using phenanthridine and xanthine as substrates, respectively. Silymarin extract (100 μg/ml)
and silibinin (100 µM) were able to inhibit the rat hepatic XO activities by 65 ± 4 and 70 ± 1%,
respectively. Furthermore, the AO activity was reduced to 62 ± 1 and 70 ± 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.