Influence of plant and bacterial myrosinase activity on the metabolic fate of glucosinolates in gnotobiotic rats

Abstract

The breakdown of glucosinolates, a group of thioglucoside compounds found in cruciferous plants, is catalysed by dietary or microbial myrosinase. This hydrolysis releases a range of breakdown products among which are the isothiocyanates, which have been implicated in the cancer-protective effects of cruciferous vegetables. The respective involvement of plant myrosinase and gut bacterial myrosinase in the conversion, in vivo, of glucosinolates into isothiocyanates was investigated in sixteen Fischer 344 rats. Glucosinolate hydrolysis in gnotobiotic rats harbouring a whole human faecal flora (Floraþ) was compared with that in germ-free rats (Flora2 ). Rats were offered a diet where plant myrosinase was either active (Myroþ) or inactive (Myro2 ). The conversion of prop-2-enyl glucosinolate and benzyl glucosinolate to their related isothiocyanates, allyl isothiocyanate and benzyl isothiocyanate, was estimated using urinary mercapturic acids, which are endproducts of isothiocyanate metabolism. The highest excretion of urinary mercapturic acids was found when only plant myrosinase was active (Flora2, Myroþ treatment). Lower excretion was observed when both plant and microbial myrosinases were active (Floraþ , Myroþ treatment). Excretion of urinary mercapturic acids when only microbial myrosinase was active (Floraþ, Myro2 treatment) was low and comparable with the levels in the absence of myrosinase (Flora2, Myro2 treatment). No intact glucosinolates were detected in the faeces of rats from the Floraþ treatments confirming the strong capacity of the microflora to break down glucosinolates. The results confirm that plant myrosinase can catalyse substantial release of isothiocyanates in vivo. The results also suggest that the human microflora may, in some circumstances, reduce the proportion of isothiocyanates available for intestinal absorption.

Hydrolysis of Glucosinolates to Isothiocyanates after Ingestion of Raw or Microwaved Cabbage by Human Volunteers

“…Only a weak relationship between the cumulative excretion of allyl isothiocyanate and phenethyl isothiocyanate was observed (r 2 ϭ 0.34). The correlation was weaker than in our previous work with rats (13,26), mainly because overall variation in urinary recovery of isothiocyanates was lower in this experiment with humans than in our previous work with rats. In previous studies with rats (13,26), we have used homologous isothiocyanates as internal standards to correct for temporal and interindividual variation in postabsorptive isothiocyanate recovery.…”

“…The correlation was weaker than in our previous work with rats (13,26), mainly because overall variation in urinary recovery of isothiocyanates was lower in this experiment with humans than in our previous work with rats. In previous studies with rats (13,26), we have used homologous isothiocyanates as internal standards to correct for temporal and interindividual variation in postabsorptive isothiocyanate recovery. In the previous work, we found that isothiocyanate recovery varied widely between individuals and occasions (between 10% and 65% of administered allyl isothiocyanate recovered) but that recovery of different isothiocyanates covaried within an individual [r 2 ϭ 94% for correlation between urinary allyl mercapturic acid output and urinary butyl mercapturic acid output after an oral dose of 50 mol to rats (13)].…”

“…Furthermore, hydrolysis of glucosinolates in the colon may yield other products in addition to isothiocyanates such as nitriles. Also, isothiocyanates produced in the colon may be subject to further degradation by the microflora (26), reducing the amount available for absorption. In vitro studies using nuclear magnetic resonance suggest amines as potential products after degradation of allyl isothiocyanate by a human fecal microflora (27).…”

Effect of cooking brassica vegetables on the subsequent hydrolysis and metabolic fate of glucosinolates

“…The low recoveries of intact glucosinolates and their metabolites in faeces of animals fed glucosinolates or isothiocyanates suggest that a substantial proportion of ingested glucosinolates and isothiocyanates are metabolised in vivo (Slominski et al 1988;Bollard et al 1997;Rouzaud et al 2003). However, there is little information on the hydrolysis of glucosinolates during and after consumption of brassica vegetables by human subjects.…”

“…The digestive and post-absorptive fate of glucosinolates in rodents and human subjects may be influenced by an array of factors such as the activity of plant myrosinase ingested, the extent of chewing, the nature of the meal containing brassica, the composition of the gut microflora and genotypic variation in post-absorptive metabolism (Shapiro et al 2001;Rouzaud et al 2003;Seow et al 2005).…”