Mass spectrometric measurement of β-endorphin and methionine enkephalin in human pituitaries. Tumors and post-mortem controls

Abstract

Two opioid neuropeptides, β-endorphin (BE), which derives from the proopiomelanocortin (POMC) precursor, and methionine enkephalin (DE), which derives from proenkephalin A, were quantified with fast atom bombardment mass spectrometry (FAB-MS) in individual human pituitaries (post-mortem) and in tumor pituitaries (post-surgery) in a study to clarify the molecular processes that occur in tumor formation. FAB-MS in the multiple reaction monitoring mode linked the precursor ion (the MH + ion) of the peptide with a fragment ion that was unique to each neuropeptide to increase significantly the molecular specificity of these quantitative analytical measurements. The ME was quantified as the intact pentapeptide, whereas BE 1–31 was quantified via its tryptic fragment BE 20–24 (NAIIK). Two corresponding stable isotope-incorporated peptides, [ 2H 5– 4Phe]-ME and [ 2H 4– 22Ile]-BE 1–31, human respectively, were used as the internal standards. The amount of each neuropeptide quantified in control post-mortem pituitaries ( n = 8) was 75.2 ± 29.6(s.e.m.) pmol ME mg −1 protein, and in the pituitary tumor samples ( n = 5), 25.0 ± 7.6 pmol ME mg −1 protein and 36.0 ± 14.8 pmol BE mg −1 protein. The difference in the BE content between the control and tumor pituitaries was significant ( p = 0.004), and reflected an aberrant metabolism of the POMC system in those human pituitary tumor tissues.