Aqueous extracts of tree peony petals: renin and angiotensin I-converting enzyme inhibitory activities in different colours and flowering stages

Tree peony (Paeonia suffruticosa Andr.) is an ornamental and medicinal plant from China. Previous studies have detected novel blood pressure-regulating substances in this species, which potentiate its value of utilization. To explore these substances, the aqueous extracts of 7 different colours of tree peony petals were assessed for inhibitory activity on renin and angiotensin-converting enzyme (ACE). The results showed that the activity of dark-coloured samples was significantly stronger than that of light-coloured ones. Furthermore, the inhibitory activity of the red tree peony petals ‘Hong TaiYang’ on renin and ACE indicated a downward trend from bud compaction to the full opening stage. The antioxidant activities of the aqueous extracts, on one side, and the correlations between phenolics and flavonoids functionalities and total contents, on the other, were also evaluated. In this regard, the extracts of different samples had ABTS free radical scavenging capacities of 17.28–210.41 mg TE per g DW, DPPH radical scavenging capacities of 35.45–150.78 mg TE per g DW, iron ion reduction capacities of 16.66–150.77 mg TE per g DW, and total phenolic content of 23.94–150.78 mg GAE per g DW. Correlation analysis revealed that the renin and ACE inhibitory activities, the DPPH and ABTS free radical scavenging capacities, and the iron reduction ability of different sample extracts were positively correlated with total phenolic contents (p < 0.01). Finally, the aqueous phenolic compounds in the sample extracts tended to show strong renin and ACE inhibitory activities and therefore exhibit a potential auxiliary blood pressure control prospect.