The Metabolic Fate of Glucoraphanin in F344 Rats

Abstract

Broccoli is the richest dietary source of the potent isothiocyanate, sulforaphane (SF). Sulforaphane has been shown to have multiple mechanisms to fight cancer. However, sulforaphane is present in broccoli as its precursor, glucoraphanin (GP). Previous studies have reported that humans do not have the enzyme to hydrolyze GP to SF and are dependent on an endogenous plant enzyme, myrosinase. Most cooking methods inactivate myrosinase. Thus when humans consume cooked broccoli they are dependent on gut microbes to hydrolyze GP to reap the health beneficial effects of SF. Relatively little is known about the hydrolysis products formed as results of bacterial action on GP and the factors that affect the conversion of GP to bioactive sulforaphane in the gut. Thus, the over all goal of this research project was to elucidate the metabolic fate of the glucosinolate, GP. We developed methods to purify GP from broccoli seed to conduct metabolism studies using rats. Rats were given purified GP by gavage or by intra-peritoneal (ip) injection; urine and feces were collected and were analyzed for metabolites using HPLC, GC and MS techniques. An important finding was that GP is absorbed when given orally and some of it is excreted unchanged in the urine. Enterohepatic recycling of absorbed GP was identified through bile analysis. A fraction (x %) of an oral GP dose was converted to SF and its metabolites. In addition to SF we also found the reduced and oxidized forms of SF, erucin and erysolin, respectively, as N-acetyl cysteine conjugates in the urine. Slightly less than half of the GP given ip was found in urine either unchanged or as its reduced from, glucoerucin. Further investigation of GP metabolism was conducted using an in vitro fermentation model. Erucin nitrile was identified as one of the fermentation product of GP. These studies show low bioavailability of dietary isothiocyanates, suggesting that there may be ways to improve bioavailability for better utilization of cruciferous vegetables in improving the public health.