Relationship between chemical structure and antioxidant activity of luteolin and its glycosides isolated from Thymus sipyleus subsp. sipyleus var. sipyleus

Abstract

The genus Thymus (Lamiaceae) is represented by about 200 species worldwide [1]. There are 39 (64 taxa) Thymus species in Turkey, 27 taxa of which are endemic [2–4]. Thymus sipyleus Boiss. subsp. sipyleus var. sipyleus is an endemic species which grows widely in Turkey [2] and is used as a spice in Turkey [5]. One triterpenic acid (ursolic acid), one phenolic acid (rosmarinic acid), and four flavonoids (luteolin, luteolin 7-O-(6″-feruloyl)-β-glucopyranoside, luteolin 5-O-β-glucopyranoside, and luteolin 7-O-β-glucuronide) were isolated from the aerial parts of T. sipyleus subsp. sipyleus var. sipyleus and identified. Afterwards, in vitro lipid peroxidation inhibition effects of the compounds were determined using TBA test methods in a bovine brain liposome system. All compounds inhibited lipid peroxidation in various degrees except for ursolic acid. The order of the lipid peroxidation activities of luteolin and its glycosides were: Luteolin 7-O-β-glucuronide > luteolin 5-O-β-glucopyranoside > luteolin 7-O-(6″-feruloyl)-β-glucopyranoside > rosmarinic acid > luteolin. However, the activity order of the compounds was completely different in DPPH radical-scavenging activity. None of the compounds show Fe chelating activity.