Composition of the Gut Microbiome Influences Production of Sulforaphane-Nitrile and Iberin-Nitrile from Glucosinolates in Broccoli Sprouts

Abstract

Isothiocyanates, such as sulforaphane and iberin, derived from glucosinolates (GLS) in cruciferous vegetables, are known to prevent and suppress cancer development. GLS can also be converted by bacteria to biologically inert nitriles, such as sulforaphane-nitrile (SFN-NIT) and iberin-nitrile (IBN-NIT), but the role of the gut microbiome in this process is relatively undescribed and SFN-NIT excretion in humans is unknown. An ex vivo fecal incubation model with in vitro digested broccoli sprouts and 16S sequencing was utilized to explore the role of the gut microbiome in SFN- and IBN-NIT production. SFN-NIT excretion was measured among human subjects following broccoli sprout consumption. The fecal culture model showed high inter-individual variability in nitrile production and identified two sub-populations of microbial communities among the fecal cultures, which coincided with a differing abundance of nitriles. The Clostridiaceae family was associated with high levels, while individuals with a low abundance of nitriles were more enriched with taxa from the Enterobacteriaceae family. High levels of inter-individual variation in urine SFN-NIT levels were also observed, with peak excretion of SFN-NIT at 24 h post broccoli sprout consumption. These results suggest that nitrile production from broccoli, as opposed to isothiocyanates, could be influenced by gut microbiome composition, potentially lowering efficacy of cruciferous vegetable interventions.

Keywords: bacteria; glucosinolate; iberin; iberin-nitrile; isothiocyanate; microbiome; sulforaphane; sulforaphane-nitrile.

Figure 1

Relative abundance of each family within the pre- and post-incubated fecal cultures (n = 10). Each color corresponds with a different family and the size of each bar corresponds to relative abundance of that family within each sample. Bars at the top represent treatment groups, pre-incubation fecal stocks (fecal stocks), post-incubation with in vitro digested broccoli (Broc), and post-incubation negative control digest (NC).

Figure 2

Principal coordinate analysis (PCoA) showing beta-diversity of samples. Each point represents one sample, with the color of the point indicating if it is a pre-incubation fecal stock or post-incubation fecal culture treated with in vitro digested broccoli (Broc), or a negative control (NC) in vitro digest. Clusters, indicated by colored ellipses, were verified by k-means clustering and the number of clusters selected by the elbow method.

Figure 3

(A) SFN-NIT and (B) IBN-NIT levels detected in fecal cultures (n = 5/cluster) following anaerobic incubation with broccoli sprouts for 24 h. Cluster corresponds to the microbial communities detected through PCoA analysis (Figure 2). Error bars show SEM. Asterisks denote statistically significant differences (p < 0.05).

Figure 4

(A) Sulforaphane-N-acetyl cysteine (SFN-NAC) and (B) sulforaphane nitrile (SFN-NIT) detected in human urine following consumption of ~200 µmol SFN equivalents in fresh broccoli sprouts directly following the 0 h time point. Colored dots represent levels of SFN metabolites in each individual subject and solid bars represent mean mass of metabolites excreted (n = 10).