Processing of proenkephalin in bovine chromaffin cells occurs in two phases

Cleavage of recombinant proenkephalin and blockade mutants by prohormone convertases 1 and 2: an in vitro specificity study

Proenkephalin (PE) derived-peptides are thought to be generated predominantly through endoproteolytic cleavage by prohormone convertases 1 and 2 (PC1 and PC2). In order to compare cleavage site preferences of these convertases, we studied the processing of recombinant wild-type rat PE and of two mutant PEs by recombinant purified mouse PC1 and PC2. Western blot analyses of timed digestions showed that both mouse PC1 and PC2 were able to produce a variety of large and intermediate sized-peptides from wild-type PE as well as from the precursors mutated at initial blockade sites. PC2 exhibited a broader specificity against PE than PC1, generating a much greater number of peptide products. Mass spectrometric identification of cleavage products showed that PC2 appeared to be the principal enzyme involved in the generation of smaller active opioids. Both enzymes were able to cleave various KR- and KK-containing sites, but PC2 was also able to cleave efficiently at an RR-V site and a KK-M site not cleaved by PC1, suggesting the exclusion of large aliphatic residues at the P1' position in PC1 cleavage. Alternative cleavage sites were readily chosen by convertases in blockade mutants, confirming in vivo results that cleavages do not follow an obligatory order. Furthermore, glycosylated PE was less efficiently processed by PC2, indicating that glycosylation may serve as a mechanism to hinder processing.

Processing of proenkephalin-A in bovine chromaffin cells. Identification of natural derived fragments by N-terminal sequencing and matrix-assisted laser desorption ionization-time of flight mass spectrometry

A large variety of proenkephalin-A-derived peptides (PEAPs) are present in bovine adrenal medulla secretory granules that are cosecreted with catecholamines upon stimulation of chromaffin cells. In the present paper, after reverse phase high performance liquid chromatography of intragranular soluble material, PEAPs were immunodetected with antisera raised against specific proenkephalin-A (PEA) sequences (PEA63-70 and PEA224-237) and analyzed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Thirty PEAPs were characterized in addition to enkephalins and whole PEA, indicating that preferential proteolytic attacks occurred at both N- and C-terminal regions. A similar approach was used to characterize PEA-derived fragments exocytotically released into the extracellular space that showed five additional minor PEAPs. Among all these naturally generated peptides, enkelytin, the antibacterial bisphos- phorylated C-terminal peptide (PEA209-237), was predominantly generated, as shown by MALDI-TOF mass spectrometry analysis, which constituted an efficient method for its identification. Finally, the data on PEA intragranular and extracellular processing in adrenal medulla are discussed in regard to the known enzymatic processing mechanisms. We note the high conservation of the cleavage points in evolutionarily diverse organisms, highlighting an important biological function for the released PEAPs.

Cardiac opioids

Opioid peptides have long been considered as neuropeptides or neurotransmitters. The more recent discovery of these same peptides in non-neuronal tissue suggests that the peptides may have autocrine, paracrine, or endocrine functions as well. The opioid peptides, enkephalins, dynorphins, and endorphins, have been found in isolated cardiac myocytes and heart tissue. This review will cover the recent literature on opioid peptides in respect to cardiac distribution, biochemistry, and function.

Characterization of antibacterial COOH-terminal proenkephalin-A-derived peptides (PEAP) in infectious fluids. Importance of enkelytin, the antibacterial PEAP209-237 secreted by stimulated chromaffin cells

Proenkephalin-A (PEA) and its derived peptides (PEAP) have been described in neural, neuroendocrine tissues and immune cells. The processing of PEA has been extensively studied in the adrenal medulla chromaffin cell showing that maturation starts with the removal of the carboxyl-terminal PEAP209-239. In 1995, our laboratory has shown that antibacterial activity is present within the intragranular chromaffin granule matrix and in the extracellular medium following exocytosis. More recently, we have identified an intragranular peptide, named enkelytin, corresponding to the bisphosphorylated PEAP209-237, that inhibits the growth of Micrococcus luteus (Goumon, Y., Strub, J. M., Moniatte, M., Nullans, G., Poteur, L., Hubert, P., Van Dorsselaer, A., Aunis, D., and Metz-Boutigue, M. H. (1996) Eur. J. Biochem. 235, 516-525). As a continuation of this previous study, in order to characterize the biological function of antibacterial PEAP, we have here examined whether this COOH-terminal fragment is released from stimulated chromaffin cells and whether it could be detected in wound fluids and in polymorphonuclear secretions following cell stimulation. The antibacterial spectrum shows that enkelytin is active against several Gram-positive bacteria including Staphylococcus aureus, but it is unable to inhibit the Gram-negative bacteria growth. In order to relate the antibacterial activity of enkelytin with structural features, various synthetic enkelytin-derived peptides were tested. We also propose a computer model of synthetic PEAP209-237 deduced from 1H NMR analysis, in order to relate the antibacterial activity of enkelytin with the three-dimensional structure. Finally, we report the high phylogenetic conservation of the COOH-terminal PEAP, which implies some important biological function and we discuss the putative importance of enkelytin in the defensive processes.

Processing site blockade results in more efficient conversion of proenkephalin to active opioid peptides

Prohormones are known to be processed at various cleavage sites in a defined temporal order, suggesting the possibility of sequential unfolding of processing sites. In order to investigate whether sequential processing at predefined sites is in fact required for proper processing, site-directed mutagenesis was performed to block known initial cleavage sites within proenkephalin. Pulse-chase/immunoprecipitation experiments were employed to analyze the fate of mutant and native proenkephalins in stably transfected AtT-20 cells. While processing did not occur at blockaded sites, surprisingly, overall processing of mutant proenkephalins proceeded efficiently, and alternative sites were chosen. When compared with native proenkephalin, processing of mutant proenkephalins occurred more slowly at early stages and more quickly at later stages. Experiments employing endoglycosidase H indicated that the early slow processing of mutant proenkephalins may be due to delays in intracellular transport. Metabolic labeling studies showed that more efficient production of bioactive opioids occurred in all processing site blockade mutants examined; these results were confirmed using several different radioimmunoassays of stored peptide products. We conclude that efficient processing of prohormone precursors does not require a specific temporal order of processing events. The fact that mutant proenkephalins were more fully processed than native proenkephalin may provide a route for more efficient production of opioid peptides in applications for chronic pain treatment.

Gene-peptide relationships in the developing rat brain: the response of preproenkephalin mRNA and [Met5]-enkephalin to acute opioid antagonist (naltrexone) exposure

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor during brain development in addition to its role as a neurotransmitter. This study examined the relationship of gene and peptide expression in the developing (postnatal day 6) rat brain by disrupting peptide-receptor interaction with either a brief (4-6 h) or continuous opioid receptor blockade using a single injection of 1 or 50 mg/kg naltrexone (NTX), respectively; such perturbations result in growth inhibition or acceleration, respectively. In the caudate putamen, an area that has completed neurogenesis by postnatal day 6 and has an abundance of PPE mRNA and enkephalins in adulthood, NTX did not influence PPE mRNA in either NTX group, or the enkephalin levels in the 1 mg/kg NTX group. [Met5]-enkephalin values in the neostriatum, however, were 67-183% greater than controls in rats given 50 mg/kg NTX, beginning 5 min after drug injection. In the cerebellum, PPE mRNA expression was depressed from 5 min to 4 h in the 1 mg/kg NTX group, and was normal thereafter; mRNA levels in the 50 mg/kg NTX group were markedly subnormal for 24 h. Enkephalin levels were significantly depressed within 5 min of drug injection and remained so for 4 h in the 1 mg/kg NTX group, but were elevated to approximately 135% of control values at 8, 16, and 24 h. Enkephalin levels were not changed in the cerebellum of the 50 mg/kg NTX group, or in the plasma of either NTX group. These data suggest that a single exposure to NTX can affect transcriptional and translational mechanisms related to PPE mRNA and opioid peptide expression in a rapid and sustained manner, and that this treatment elicits a specific pattern of alterations dependent upon the brain region sampled, drug dosage, and/or the duration of opioid receptor blockade. Additionally, our results indicate that the decreased DNA synthesis in external germinal cells occurring after opioid receptor blockade as recorded earlier may be related to an increase in the potent opioid growth factor, [Met5]-enkephalin.

Alteration of opioid peptide concentrations in the rat pituitary following survivable closed head injury

Concentration changes of methionine enkephalin-like immunoreactivity (ME-li) and beta-endorphin-like immunoreactivity (BE-li) in the rat pituitary following diffuse brain injury were studied. Closed head injury was induced by a weight-drop trauma device (450 g x 2 m). The level of closed head injury used in this study altered the pituitary opioid peptide concentrations. The level of ME-li did not change in the experimental groups 3 hours, 10 hours, 24 hours, and 3 days after the trauma, but significantly increased by 34% 10 days after the trauma. BE-li remained constant 3 hours and 10 hours following the injury, increased by 48% at 24 hours, and remained at this level for 10 days after the trauma (44% at 3 days, and 40% at 10 days). The levels of ME-li and BE-li in the control sham-operated rats did not change during these times. The present measurements of BE-li and ME-li in the pituitary indicate that the opioid peptides that derive from two different neuropeptidergic systems, proopiomelanocortin (POMC) and preproenkephalin A, respectively, may participate in the pathophysiology of a closed head injury.