Aromatic profiles and 15-LOX inhibitory effects of Australian and New Zealand Manuka honeys

Manuka ( Leptospermum ) honeys from Australia (Aust) and New Zealand (NZ) contain high levels of aromatic compounds, though only a subset are true phenolics in that they contain a hydroxyl group attached to an aromatic ring. Consequently, the traditional Folin-Ciocalteu Total Phenolic Content (FC-TPC) assay fails to detect many non-phenolic aromatic compounds that may confer health benefits. This paper quantified both phenolic and non-phenolic aromatic compounds in monofloral manuka honeys using high-performance liquid chromatography with diode array detection (HPLC-DAD) and the FC-TPC assay. In addition, this study evaluated the capacity of the honeys to inhibit 15-lipoxygenase (15-LOX), an enzyme that causes peroxidation of long chain polyunsaturated fatty acids. Australian (n = 7) and New Zealand (n = 6) Leptospermum scoparium ( L. scoparium ) honeys showed the highest total aromatic concentrations by HPLC-DAD (>2100 mg/kg), significantly exceeding those of L. liversidgei (n = 5), L. whitei (n = 5), and L. speciosum (n = 3) honeys. The FC-TPC assay identified higher phenolic levels in L. scoparium and L. polygalifolium honeys than in the other species studied. Sugar free Australian and New Zealand manuka honey aromatic extracts were potent inhibitors of 15-LOX enzyme, with IC 50 values recorded at <15% of stock solutions (total extractables from 20.0 g of honey dissolved in 1.0 mL of 50% aqueous ethanol). Australian (IC 50 , 13.2 ± 0.7%) and New Zealand (13.1 ± 1.9%) L. scoparium honeys were more potent in inhibiting 15-LOX than L. liversidgei (IC 50 , 37.5 ± 6.3%) and L. speciosum (IC 50 , 36.6 ± 10.5%). Overall, this study demonstrates that HPLC-DAD provides a more comprehensive assessment of aromatic compounds in manuka honey than the FC-TPC assay and highlights the potential contribution of these compounds to anti-inflammatory activity.