MASS-SPECTROMETRIC MEASUREMENT OF BETA-ENDORPHIN AND METHIONINE ENKEPHALIN IN HUMAN PITUITARIES - TUMORS AND POSTMORTEM CONTROLS

Two opioid neuropeptides, beta-endorphin (BE), which derives from the proopiomelanocortin (POMC) precursor, and methionine enkephalin (ME), 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 BE1-31 was quantified via its tryptic fragment BE20-24 (NAIIK). Two corresponding stable isotope-incorporated peptides, [H-2(5)-4Phe]-ME and [H-2(4)-22Ile]-BE1-31,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 132.5 +/- 22.3 (s.e.m.) pmol BE 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.