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Cation-exchange high-performance liquid chromatography (HPLC) was used to increase the sensitivity and specificity of the radioimmunoassay of plasma beta-endorphin. Proteins were precipitated from a 0.5 to 2.5 ml sample of plasma with 60% acetonitrile at pH 4.7. The supernatant was subjected to cation-exchange HPLC. Gradient elution with volatile buffers was used to separate beta-endorphin from beta-lipotropin. The beta-endorphin fraction (1.8 ml) was concentrated by lyophilization and subjected to radioimmunoassay. In healthy pregnant women at labour plasma concentration of beta-endorphin varied from 105 to 403 pg/ml. In healthy non-pregnant women plasma concentration of beta-endorphin was low, exceeding the detection limit (4 pg/ml) of the assay in only one of the 7 subjects studied.
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