Abstract
We report for the first time that morphine-6-glucuronide, a highly analgesic morphine-derived molecule, is present in adrenal chromaffin granules and secreted from chromaffin cells upon stimulation. We also demonstrate that phosphatidylethanolamine-binding protein (alternatively named Raf-1 kinase inhibitor protein or RKIP) acts as an endogenous morphine-6-glucuronide-binding protein. An UDP-glucuronosyltransferase 2B-like enzyme, described to transform morphine into morphine-6-glucuronide, has been immunodetected in the chromaffin granule matrix, and morphine-6-glucuronide de novo synthesis has been characterized, demonstrating the possible involvement of intragranular UDP-glucuronosyltransferase 2B-like enzyme in morphine-6-glucuronide metabolism. Once secreted into the circulation, morphine-6-glucuronide may mediate several systemic actions (e.g. on immune cells) based on its affinity for mu-opioid receptors. These activities could be facilitated by phosphatidylethanolamine-binding protein (PEBP), acting as a molecular shield and preventing morphine-6-glucuronide from rapid clearance. Taken together, our data represent an important observation on the role of morphine-6-glucuronide as a new endocrine factor.
Highlights
For 20 years, cerebral endogenous morphine has been isolated and characterized, and its structure has been shown to be identical to morphine from poppy [1, 2]
We demonstrate that phosphatidylethanolamine-binding protein acts as an endogenous morphine-6-glucuronide-binding protein
Immunolabeling of Morphine-like Components in Chromaffin Cells— We extended our previous observations related to the presence of morphine and its derivatives in adrenal medulla and PC-12 cells [15, 16] by investigating the subcellular localization of morphine and its derivatives in chromaffin cells by laser confocal microscopy
Summary
For 20 years, cerebral endogenous morphine has been isolated and characterized, and its structure has been shown to be identical to morphine from poppy [1, 2]. Several crucial steps were reached when Zenk and co-workers [10, 11] demonstrated that morphine can be formed using a multistep biosynthetic route, and Zhu et al [12] showed that human primary polymorphonuclear cells are able to synthesize morphine These authors have shown morphine de novo synthesis in human and animal primary and cancer cell cultures. Chromaffin cells are neuroendocrine cells originating from the neural crest and are the predominant cell type in the adrenal medulla (for review, see Ref. 13) These cells possess the catecholamine biosynthetic pathway, leading to dopamine and adrenaline/noradrenaline synthesis [13]. M6G de novo synthesis was demonstrated using the UGT2B-like enzyme present in chromaffin granules
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