Demonstration and characterization of immunoreactive methionine-enkephalin, leucine-enkephalin, methionine-enkephalin-Arg6-Gly7-Leu8 and methionine-enkephalin-Arg6-Phe7 in human phaeochromocytoma

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Abstract
To elucidate whether or not human pheochromocytoma contains methionine-enkephalin-Arg6-Gly7-Leu8 (Met-enkephalin-Arg-Gly-Leu) and methionine-enkephalin-Arg6-Phe7 (Met-enkephalin-Arg-Phe) together with methionine-enkephalin (Met-enkephalin) and leucine-enkephalin (Leu-enkephalin), all of which are known to exist in the same precursor molecule (preproenkephalin A), extracts from 16 pheochromocytomas were examined using high performance liquid chromatography (HPLC) and gel exclusion chromatography coupled with radioimmunoassays (RIA) for these 4 opioid peptides. Met-enkephalin-Arg-Gly-Leu-like immunoreactivity (-LI) and Met-enkephalin-Arg-Phe-LI existed together with Met-enkephalin-LI and Leu-enkephalin-LI in 16 pheochromocytomas. There was a wide variation in contents of Met-enkephalin-LI, Leu-enkephalin-LI, Met-enkephalin-Arg-Gly-Leu-LI and Met-enkephalin-Arg-Phe-LI. Significant positive correlations were observed among the contents of these 4 opioid peptides in 16 pheochromocytomas. HPLC and gel exclusion chromatography followed by the RIA showed the presence of Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Gly-Leu and Met-enkephalin-Arg-Phe together with their high MW forms, which existed in variable amounts. Molar ratios of the contents of these 4 opioid peptides determined after HPLC varied from case to case. The co-existence of Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Gly-Leu and Met-enkephalin-Arg-Phe in human pheochromocytomas, suggesting the preservation of amino acid sequences of thes 4 opioid peptides even in neoplastic tissue, is thus indicated. Evidence for some abnormalities in the posttranslational processing of preproenkephalin A to these 4 opioid peptides was obtained in certain pheochromocytomas.