Figure 1.

During chewing of raw broccoli sprouts, plant myrosinase converts glucoraphanin (GRP) to sulforaphane (SFN) in the mouth. In the gut lumen, GRP is further converted to either SFN or sulforaphane-nitrile (SFN-NIT) by the gut microbiome. It is unclear if microbes conjugate SFN to glutathione (GSH), however, only free SFN is taken up into enterocytes. Within the enterocyte, SFN-GSH is either excreted back into the gut lumen or enters circulation where it is exists as SFN-GSH or in its free form which conjugates with blood proteins. SFN is transported to either tissues where it exerts its bioactivity or to the liver where it is metabolized via the mercapturic acid pathway. Mercapturic acid metabolites, SFN-cysteine (SFN-Cys) and SFN-N-Acetyl-Cysteine (SFN-NAC), from the liver are either exported to the bile for excretion to the feces, or back into the blood to go to tissues. SFN also goes to the kidney where it is converted to SFN-NAC and excreted into the urine. Un-hydrolyzed GRP is either excreted into the feces or absorbed where it is either transported to the kidneys to be excreted in urine or to the liver where it is excreted into bile. GRP that undergoes enterohepatic circulation and is either hydrolyzed in the gut or excreted to the feces. SFN-NIT is absorbed from the gut lumen where it is transported to kidneys for excretion into urine. SFN-NIT metabolism in humans needs further investigation.