Thermal stability, and in vitro and in vivo metabolism of four main glucosinolates in Isatis indigotica roots

The glucosinolates (GLSs) and their degradation products (such as goitrin) in Isatis indigotica roots have been the subject of considerable research interest due to their antiviral activities. The degradation mechanism and products of GLSs under the endogenous myrosinase are well-established. Nevertheless, it remains uncertain whether GLSs in I. indigotica roots can undergo thermal decomposition, or whether the two principal GLSs (progoitrin and epiprogoitrin) can biotransform into the bioactive goitrin in the absence of myrosinase. In this study, the thermal stability and metabolic behavior of four main GLSs (including progoitrin, epiprogoitrin, gluconapin, and neoglucobrassicin) were estimated for the first time. Gluconapin demonstrated the most robust thermal stability among the four GLSs, retaining its structural integrity even at 100 °C. The remaining three GLSs were found to be stable only at lower temperatures (0–20 °C), exhibiting degradation at 40 °C and complete degradation at 60 °C. The degraded products of the three GLSs were subjected to analysis by static headspace GC–MS, which were identified as isothiocyanates, thiocyanates, nitriles, and enols. The four GLSs were not biotransformed into their respective products by simulated gastric fluid, simulated intestinal fluid, rat liver microsomes, or human intestinal flora. The in vivo metabolism study of the aliphatic glucosinolates (progoitrin, epiprogoitrin, gluconapin, and sinigrin) in mice revealed that the intact GLS and their metabolites of these four GLSs were undetected in the mice plasma by UPLC-ESI-MS/MS, indicating that the GLS were poorly absorbed into the blood. However, only a common minor metabolite of the four GLSs were identified in the urine of the mice, along with abundant intact GLSs. The metabolite was identified as N-acetylcysteine conjugates by LC-MS, with the aid of the reported mercapturic acid metabolic pathway of GLSs. The findings of this study indicated that the GLSs are predominantly excreted in urine in their original forms and are not absorbed into the bloodstream. In conclusion, the two principal GLSs (progoitrin and epiprogoitrin) in I. indigotica roots are unable to undergo conversion to active goitrin under conditions of heat stress or physiological stress in the absence of myrosinase.