Plant biophenols have been shown to be effective in the modulation of Alzheimer's disease (AD) pathology resulting from free radical-induced oxidative stress and imbalance of the redox chemistry of transition metal ions (e.g., iron and copper). On the basis of earlier reported pharmacological activities, olive biophenols would also be expected to have anti-Alzheimer's activity. In the present study, the antioxidant activity of individual olive biophenols (viz. caffeic acid, hydroxytyrosol, oleuropein, verbascoside, quercetin, rutin and luteolin) were evaluated using superoxide radical scavenging activity (SOR), hydrogen peroxide (H2O2) scavenging activity, and ferric reducing ability of plasma (FRAP) assays. The identification and antioxidant activities in four commercial olive extracts-Olive leaf extractTM (OLE), Olive fruit extractTM (OFE), Hydroxytyrosol ExtremeTM (HTE), and Olivenol plusTM (OLP)-were evaluated using an on-line HPLC-ABTS•+ assay, and HPLC-DAD-MS analysis. Oleuropein and hydroxytyrosol were the predominant biophenols in all the extracts. Among the single compounds examined, quercetin (EC50: 93.97 μM) and verbascoside (EC50: 0.66 mM) were the most potent SOR and H2O2 scavengers respectively. However, OLE and HTE were the highest SOR (EC50: 1.89 μg/mL) and H2O2 (EC50: 115.8 μg/mL) scavengers among the biophenol extracts. The neuroprotection of the biophenols was evaluated against H2O2-induced oxidative stress and copper (Cu)-induced toxicity in neuroblastoma (SH-SY5Y) cells. The highest neuroprotection values (98% and 92%) against H2O2-induced and Cu-induced toxicities were shown by the commercial extract HTETM. These were followed by the individual biophenols, caffeic acid (77% and 64%) and verbascoside (71% and 72%). Our results suggest that olive biophenols potentially serve as agents for the prevention of neurodegenerative diseases such as AD, and other neurodegenerative ailments that are caused by oxidative stress.

The focus of this study was on inhibition of enzymes involved in the pathogenesis Alzheimer's disease (AD) including prime amyloid beta (Aβ) producing enzyme (β-secretase: BACE-1) and disease progression enzymes including acetylcholinesterase (AChE), butyrylcholinesterase (BChE), histone deacetylase (HDAC), and tyrosinase along with the catecholamine L-DOPA, by using olive biophenols. Here we report the strongest inhibition of BACE-1 from rutin (IC₅₀: 3.8 nM) followed by verbascoside (IC₅₀: 6.3 nM) and olive fruit extract (IC₅₀: 18 ng), respectively. Olive biophenol, quercetin exhibited strongest enzyme inhibitory activity against tyrosinase (IC₅₀: 10.73 μM), BChE (IC₅₀: 19.08 μM), AChE (IC₅₀: 55.44 μM), and HDAC (IC₅₀: 105.1 μM) enzymes. Furthermore, olive biophenol verbascoside (IC₅₀: 188.6 μM), and hydroxytyrosol extreme extract (IC₅₀: 66.22 μg) were showed the highest levels of inhibition against the HDAC enzyme. Neuroprotective capacity against levodopa-induced toxicity in neuroblastoma (SH-SY5Y) cells of olive biophenols were assessed, where rutin indicated the highest neuroprotection (74%), followed by caffeic acid (73%), and extract hydroxytyrosol extreme (97%), respectively. To the best of our knowledge, this is the first in vitro report on the enzymes inhibitory activity of olive biophenols. Taken together, our in vitro results data suggest that olive biophenols could be a promising natural inhibitor, which may reduce the enzyme-induced toxicity associated with the oxidative stress involved in the progression of AD. Chemical compounds used in the study: Acetylthiocholine iodide (PubChem CID: 74629); S-Butyrylthiocholine chloride (PubChem CID: 3015121); Caffeic acid (PubChem CID: 689043); Dimethyl sulfoxide (DMSO) (PubChem: 679); L-3,4-Dihydroxyphenylalanine (L-DOPA) (PubChem CID: 6047); 5,5′-Dithiobis (2-nitrobenzoic acid) (DTNB) (PubChem CID: 6254); Epigallocatechin gallate (EGCG) (PubChem CID: 65064); Ethylenediamine tetraacetic acid (EDTA) (PubChem CID: 6049); Galantamine hydrobromide (PubChem CID: 121587); LGlutamine (PubChem CID: 5961); Hydroxytyrosol (PubChem CID: 82755); Kojic acid (PubChem CID: 3840); Luteolin (PubChem CID: 5280445); Oleuropein (PubChem CID: 5281544); Penicillin-streptomycin (PubChem CID: 131715954); Quercetin (PubChem CID: 5280343); Rutin (PubChem CID: 5280805); Tris-HCl buffer (PubChem: 93573); Trypan blue (PubChem: 9562061).