Quantification of endogenous alpha- and gamma-endorphins in rat brain by liquid chromatography-tandem mass spectrometry.

Quantification of alpha- and gamma-endorphins in rat brain using liquid chromatography-electrospray ionization-tandem mass spectrometry is described. [D-Ala(2)]-gamma-endorphin is used as an internal standard. The precursor-to-product ion MRM transitions for alpha-endorphin, gamma-endorphin, and [D-Ala(2)]-gamma-endorphin were m/z 873.6-->429.6; 929.6-->542.3; 936.6-->542.3, respectively. The method was validated in terms of linearity, specificity, sensitivity, recovery, precision, and accuracy. The assay was linear over a concentration range of 0.1-200 ng/mL with the limit-of-detection of 0.03 ng/mL and limit-of-quantification of 0.1 ng/mL. The endogenous concentrations of alpha- and gamma-endorphins in rat brains were 13.8+/-0.57 (mean+/-SD; n=5) and 2.5+/-0.43 ng/g of wet tissue weight, respectively.