Bioavailability and biological effects of isothiocyanates from broccoli sprout powder

Abstract

Broccoli sprout powder (BSP) is a rich source of glucosinolates that can be hydrolysed to isothiocyanates by enzyme myrosinase when the powder comes into contact with water. The major isothiocyanate produced in broccoli is sulforaphane. While several studies have shown sulforaphane can protect cells from oxidative stress in vitro, there is limited information about beneficial effects to humans consuming BSP. Formation of isothiocyanates is optimal at neutral pH; under acidic conditions competing reactions can occur decreasing isothiocyanate bioavailability. In this study, I showed lowering the pH of the solution containing BSP to pH 1.2 significantly lowered isothiocyanate generation. This suggests that delivery of BSP in capsules that degrade in the stomach will compromise isothiocyanate bioavailability. Experiments were performed to test a gastro-resistant enteric polymer coated-capsule. BSP alone and standard gelatin encapsulated powder were fed to healthy volunteers. There was comparable excretion of isothiocyanate metabolites in urine. Enteric-coated capsules showed a delay in excretion of isothiocyanates, indicating protection from disintegration in the stomach, however, isothiocyanate metabolite level was significantly reduced. This suggests that capsule breakdown was too slow. Further optimization with alternative enteric-coating formulations is required. Isothiocyanates provide protection against oxidative stress by increasing the expression of antioxidant enzymes through the Kelch-like ECH-associated protein 1/nuclear factor erythroid 2-related factor 2/antioxidant response element (Keap1/Nrf2/ARE) pathway. Upregulation of Nrf2 in lymphocytes isolated from peripheral blood was detected when these cells were treated with sulforaphane or BSP extract in vitro. However, there was no detectable increase in Nrf2 protein levels in lymphocytes isolated from volunteers three hours post-ingestion of BSP. In an attempt to develop an alternate marker of efficacy, a Seahorse Extracellular Flux Analyser was used to measure mitochondrial bioenergetics in monocytes and lymphocytes obtained from human blood. Cells were treated with hydrogen peroxide (H2O2) and it was discovered that mitochondrial reserve capacity was the parameter most sensitive to H2O2. The susceptibility of mitochondria to oxidative stress was compared in lymphocytes and monocytes isolated from several different volunteers and showed variation within and between individuals tested on different days. Ingestion of BSP protected mitochondrial reserve capacity from oxidative stress in cells harvested at six hours post-ingestion compared to those isolated prior to ingestion. These results indicate a larger study is warranted, and that this methodology for assessing the resilience of cells to oxidative stress has promise for investigating the impact of diet, exercise, age and disease.