The biotransformation of glucosinolates : a bacterial perspective : a thesis presented in partial fulfilment of the requirements for the degree of PhD in Food Technology at Massey University, Manawatū, New Zealand
Epidemiological studies have shown an association between the
consumption of cruciferous vegetables and a reduced risk of certain types of
cancers, in particular, colon, bladder and bowel. This is thought to be due to the
conversion of glucosinolates present in the vegetables into bioactive
isothiocyanates which in turn stimulate a host response involving detoxification
pathways. Conversion of glucosinolates is catalysed by the enzyme myrosinase,
which is co-produced by the plant but stored in separate tissue compartments and
brought together when the tissue is damaged. Myrosinase activity can be reduced
or lost during storage of vegetables and is often inactivated by cooking. However,
in the absence of active plant myrosinase, bacteria are capable of carrying out a
myrosinase-like activity on glucosinolates producing isothiocyanates or nitriles.
This thesis examined the bacterial biotransformation of glucosinolates by
two lactic acid bacteria and Escherichia coli Nissle 1917, all three considered
beneficial bacteria. They were compared with a known glucosinolate-metabolising
gut bacterium Enterobacter cloacae in vitro, in vivo and ex vivo to determine the
bacterial responses to glucosinolates and what the products of their glucosinolate
metabolism might be. Exposure of the host to beneficial bacteria and
glucosinolates resulted in induction of the host detoxification enzyme quinone
reductase which was elevated in bladder tissue for all dietary intervention groups
consuming glucosinolates and beneficial bacteria, alone or combined.
In vitro, Nissle reduced alkylsulfinyl glucosinolates and their hydrolysis
products through redox to alkylthiols and in vivo, the host microbiota responded
similarly. In vivo, the host response to alkylthiol nitriles was to oxidise these back
again to alkylsulfinyl nitriles and oxidise further resulting in some nitriles being
irreversibly oxidised to the sulfone.
The association between consumption of cruciferous vegetables and reduced
cancer of the colon, bladder and bowel is only that; an association. However, the
results of this thesis demonstrated that bladder tissue was affected by beneficial
bacteria and glucosinolates alone or together, which suggests that both exert a
protective effect that could be measured by elevated quinone reductase, a
biomarker for cancer chemoprevention.