Unique fluorescence and high-molecular weight characteristics of protein isolates from manuka honey (Leptospermum scoparium)

- Fluorescence intensity of manuka honey protein is higher than non-manuka protein.
- Fluorescence intensity depends on methylglyoxal (MGO) content of honey.
- Protein cross-linking in manuka honey is caused by MGO.
- Non-manuka honey proteins are mainly crosslinked by disulfide bonds.
- Unique characteristics of manuka honey proteins cannot be simulated by MGO addition.

This study compared the fluorescence properties (λex/em = 350/450 nm) and molecular size of proteins from manuka and non-manuka honey. The fluorescence characteristics of non-manuka and manuka proteins differ markedly, whereby manuka honey protein fluorescence increases with increasing methylglyoxal (MGO) content of the honey. It was concluded that manuka honey proteins are modified due to MGO-derived glycation and crosslinking reactions, thus resulting in fluorescent structures. The molecular size of honey proteins was studied using size exclusion chromatography. Manuka honey proteins contain a significantly higher amount of high molecular weight (HMW) fraction compared to non-manuka honey proteins. Moreover, HMW fraction of manuka honey proteins was stable against reducing agents such as dithiothreitol, whereas HMW fraction of non-manuka honey proteins was significantly decreased. Thus, the chemical nature of manuka honey HMW fraction is probably covalent MGO crosslinking, whereas non-manuka HMW fraction is caused by disulfide bonds. Storage of a non-manuka honey, which was artificially spiked with MGO and DHA, did not induce above mentioned fluorescence properties of proteins during 84 days of storage. Hence, MGO-derived fluorescence and crosslinking of honey proteins can be useful parameters to characterize manuka honey.

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