Identification and measurement of oxindole (2-indolinone) in the mammalian brain and other rat organs.

Oxindole, a putative tryptophan metabolite able to cause profound sedation when administered in relatively low doses to mammals, has been identified and measured in the brains of mice, rats, and guinea pigs using HPLC and GC/MS with a quadrupole ion trap and a collision-induced dissociation mass spectrometer. The identification and measurement of the compound required a protein precipitation step with HClO4, extraction into chloroform, an HPLC separation on a reverse-phase column, and detection by UV or coulometry. The definitive identification of the oxindole peak was obtained with a Saturn 4D GC/MS quadrupole ion trap operated under GC/MS, GC/MS/MS, and GC/MS/MS/MS modes. The HPLC methods we used had a low interassay variability, easily allowing the identification and measurement of the compound in 1 g of tissue. The oxindole concentrations in rat brain, blood, liver, and kidney were each approximately 100 pmol/g wt. Interestingly, the content of oxindole in the guinea pig brain was found to be significantly lower than that in the mouse and rat brains, possibly reflecting a lower dietary intake of tryptophan in the guinea pigs.