Synergistic Enhancement of Antioxidant Activity in <i>Arbutus unedo</i> Leaf Extract by Biological Fluids: Implications for Functional and Nutraceutical Applications

This study examined the effect of biological fluids (saliva, blood plasma, and urine) on the antioxidant capacity of an aqueous extract derived from Arbutus unedo leaves. The extract displayed a concentration-dependent reducing activity in the ferric reducing power assay (IC₅₀ = 292 ± 7.54 µg/mL) and phosphomolybdenum assays (IC₅₀ = 461.67 ± 4.16 µg/mL), indicating considerable antioxidant potential, albeit inferior to ascorbic acid (IC₅₀ = 56.72 ± 2.79 and 156.55 ± 7.40 µg/mL, respectively). The investigation via high-performance liquid chromatography-mass spectrometry (HPLC-MS) discovered significant phenolic substances, such as quinic acid, arbutin, epicatechin, and quercetin derivatives, which likely contribute to the reported effects. The interaction with biological fluids enhanced the antioxidant activity synergistically. Saliva (50 µL) enhanced molybdenum reduction by 10%, but plasma (25 µL) augmented it by 27.8% relative to the extract alone. Urine (25 µL) exhibited the most significant effect, increasing molybdenum reduction by 30.24%. Likewise, iron reduction was enhanced by 32.00% with plasma and 19.00% with urine. The observed effects were ascribed to interactions between polyphenols and proteins or endogenous antioxidants, such as uric acid, in bodily fluids. The results underscore the significance of physiological context in assessing plant-based antioxidants, indicating that A. unedo extract may exhibit increased bioactivity in vivo due to synergistic interactions with biological components. The study supports the potential of A. unedo as a functional food ingredient and highlights the importance of considering biological fluid interactions during nutraceutical formulation.