In vitro inhibition of rat monoamine oxidase by liquiritigenin and isoliquiritigenin isolated from Sinofranchetia chinensis

AIM: To study the inhibition of liquiritigenin (1) and isoliquiritigenin (2 ) isolated from Sinofranchetia chinensis on rat monoamine oxidase A and B ( MAO A and B). METHODS: Rat brain mitochondrial fraction, prepared by differ ential centrifugation, was utilized as a source of MAO activity. MAO activi ty was determined radiochemically with [C-14] 5-hydroxytryptamine (5-HT) an d [C-14]beta-phenylethylamine (beta-PEA) used as MAO A or B specific radiol abled substrates, respectively. The K-i and K-I values were obtained from L ineweaver-Burk plot using linear regression analysis. RESULTS: Liquiritigen in and isoliquiritigenin were found to be inhibitory against both MAO A and B in a dose-dependent manner. IC50 (95 % of confidence limits) of liquirit igenin and isoliquiritigenin were 32 (26 - 36) and 13.9 (12.8-15.6) mu mol/ L for the inhibition of MAO A, and 104.6 (89.0 - 118.9) and 47.2 (39.5 - 54 .5) mu mol/L for that of MAO B, respectively. Lineweaver-Burk transformatio n of the MAO A inhibition data indicated that the inhibition was non-compet itive for both Liquiritigenin and isoliquiritigenin whereas their inhibitio n of MAO B was of mixed type. Regarding MAO A inhibition, the K-i values of liquiritigenin and isoliquiritigenin were 31.5 mu mol/L and 14.3 mu mol/L, respectively. As to the inhibition of MAO B, the K-i and K-I data for liqu iritigenin were 164.7 and 15.2 mu mol/L, and those for isoliquiritigenin we re 62.2 and 9.3 mu mol/L, respectively. CONCLUSION: Liquiritigenin and isol iquiritigen inhibited the activity of MAO A and B in rat brain mitochondria , and the latter was more active than the former.