Enzymatic, chemical, and thermal breakdown of H-3-labeled glucobrassicin, the parent indole glucosinolate

The enzymatic, chemical, and thermal breakdown pathways of glucobrassicin, the major indolylmethyl glucosinolate of cruciferous vegetables, have been studied using synthetic H-3-labeled glucobrassicin (GBS). Radio-HPLC was used to analyze qualitatively and quantitatively the resulting products as well as their kinetics of formation. Enzymatic breakdown of GBS under myrosinase action gave rise to different indole compounds [indole-3-carbinol (I3C), indole-3-acetonitrile (IAN) and 3,3'-diindolylmethane (DIM)]. At neutral pH, GBS degradation was almost complete after 1 h, and the major breakdown product was I3C, which could be converted to DIM. The formation of this self-condensation product was observed as photosensitive. In acidic conditions, enzymatic degradation of GBS was a slower phenomenon, requiring 24 h to be nearly complete. IAN and I3C were the only two products occurring, and it was observed that the light had no effect either on the rate of formation or on the relative proportions of the breakdown products observed. GBS appeared as a very stable compound since no chemical degradation could be observed after 2 h in different aqueous media with pH in the 2-11 range. Moreover, after exposure to heat treatment, GBS was weakly degraded (10% in 1 h), giving risk to a new minor indole condensation product corresponding to a 3-(indolylmethyl)glucobrassicin (IM-GBS).