Inhibition of renin-angiotensin system enzymes by leafy vegetables polyphenol extracts related to fertilizer micro-dosing and harvest time

Increase in human population coupled with continuous rise in oxidative stress-related diseases requires availability of healthier foods like vegetables, which are polyphenol-rich sources. Polyphenols are able to inhibit angiotensin-I converting enzyme (ACE) and renin, thereby regulating blood pressure but this ability can be influenced by nutrient management during plant growth. Fertilizer micro-dosing is a valuable nutrient management option, which increases crop productivity with minimal expenditure of resources. This study assessed the effects of fertilizer micro-dosing and harvest times on inhibition of ACE and renin activities by aqueous polyphenol extracts from Amaranthus cruentus, Ocimum gratissimum and Solanum macrocarpon leaves. Different levels of urea (0, 20, 40 and 60 kg ha^-1) with cow manure (5 t ha^-1) or without manure (80 kg ha^-1) were used to grow the leafy vegetables before they were harvested three times at two (A. cruentus) and four weeks intervals (O. gratissimum and S. macrocarpon). Polyphenols were extracted from leaf powders using water (1:20 ratio) and total phenolic content (TPC), total flavonoids content (TFC), ACE and renin inhibition were assayed according to standard protocols. Two-way analysis of variance and Duncan's test (p<0.05) revealed that TPC (605.70±39.51 to 1024.83±102.89 mg g^-1; gallic acid equivalent) and TFC (149.02±29.12 to 276.19±63.30 mg g^-1; rutin equivalent) were not affected by fertilizer doses and harvest times except TPC from O. gratissimum. Inhibitions of ACE (0.25±0.01 to 8.46±0.42 mg mL^-1) and renin (-95.41±2.25 to 36.42±0.22%) activities by each specie were significantly affected by fertilizer dose and harvest time. Production with cow manure combined with low or medium doses of urea (20 or 40 kg ha^-1) followed by collection at the first two harvests are recommended for polyphenol extracts with potential inhibition of renin-angiotensin system enzymes.