Proenkephalin A processing in the upper digestive tract: isolation and characterisation of phosphorylated N-terminally extended Met-enkephalin Arg6Phe7 variants

Enkephalin related peptides are released from jejunum wall by orally ingested bromelain

Stem bromelain [EC 3.4.22.32] is a thiol-endopeptidase and orally recommended in traditional medicine due to its analgesic activity, but the mechanisms are not known. Proenkephalin is expressed in the nervous system, but also in the gastrointestinal tract, where it can be assessed by ingested stem bromelain. Here we demonstrated that stem bromelain hydrolyses synthetic proenkephalin fragments after basic amino acid residues flanking the enkephalin sequences. We also observed with in vivo studies that oral administration of bromelain reduced jejunum proenkephalin levels and increased the serum enkephalin in mice. Effective anti-nociceptive effects in mice were observed 3 h after oral administration of 3 mg/kg stem bromelain by the acetic acid-induced writhing test. However, with higher doses this effect is reduced due to hydrolysis of enkephalin that possibly occurs by the presence of ananain in commercial pineapple stem bromelain preparations, that is also a thiol-protease with broad specificity. The analgesic effects were also evaluated by hot-plate and formalin tests and the obtained results indicated that enkephalin generated in intestine acts in periphery where it also can have anti-inflammatory activity.

FGF23 is endogenously phosphorylated in bone cells

Levels of serum phosphate are controlled by the peptide hormone FGF23, secreted from bone osteocytes. Elevated levels of circulating FGF23 are a key factor in several hypophosphatemic disorders and play a role in chronic kidney disease. Posttranslational processing of FGF23 includes multi-site O-glycosylation, which reduces intracellular cleavage by proprotein convertases. The FGF23 protein also contains four serine phosphorylation consensus sequences (S-X-D/E); in this work, we asked whether FGF23 is a substrate for secretory phosphorylation. Both HEK cells as well as IDG-SW3 cells, an osteocyte model, incorporated radiolabeled orthophosphate into intact FGF23, as well as into the 14-kDa carboxy-terminal-but not the 17-kDa N-terminal-fragment. Sequential serine-to-alanine site-directed mutagenesis of four kinase consensus sites showed that labeling occurred on three serines within the carboxy-terminal fragment, Ser180 (adjacent to the cleavage site), Ser207, and Ser212. Liquid chromatography-coupled mass spectroscopy indicated the presence of phosphate at Ser212 in recombinant R&D mouse FGF23(R179Q) , confirming labeling results. A phosphopeptide-specific antibody was raised against phospho-Ser212 and exhibited immunoreactivity in osteocytes present in mouse long bone, providing further evidence that FGF23 is naturally phosphorylated in bone. Bone SIBLING proteins are serine-phosphorylated by the ubiquitous Golgi secretory kinase FAM20C. Cotransfection of HEK and MC3T3 cells with FGF23 and active, but not inactive, FAM20C kinase increased the storage and release of FGF23 in radiolabeling experiments, indicating potential effects of phosphorylation on FGF23 stability. Collectively, these data point to an important role for phosphorylation of FGF23 in bone.

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.

The C-terminal bisphosphorylated proenkephalin-A-(209-237)-peptide from adrenal medullary chromaffin granules possesses antibacterial activity

The chromaffin granules have been shown to be an excellent model to study the processing of proenkephalin-A and chromogranins. Recently, we reported a study dealing with the processing of chromogranin B/secretogranin I and the occurrence of the C-terminal chromogranin B-derived peptide 614-626 which was shown to have antibacterial activity [Strub, J.M., Garcia-Sablone, P., Looning, K., Taupenot, L., Hubert, P., Van Dorsselaer, A., Aunis, D. & Metz-Boutigue, M.H. (1995) Eur. J. Biochem. 229, 356-368]. We also observed that this new antibacterial activity present in chromaffin granules was associated with other endogenous protein-derived fragments yet to be characterized. The present study reports the isolation and characterization of a peptide which possesses antibacterial activity and which corresponds to the C-terminal 209-237 sequence of proenkephalin-A. A detailed study using microsequencing and matrix-assisted-laser-desorption time-of-flight mass spectrometry (MALD-TOF MS) allowed us to correlate the antibacterial activity of this peptide named enkelytin (FAEPLPSEEEGESYSKEVPEMEKRYGGFM) with post-translational modifications. Endogenous bisphosphorylated proenkephalin-A-(209-237) was active on Micrococcus luteus and Bacillus megaterium killing bacteria in the 0.2 - 0.4 microM range but was inactive in similar conditions towards Escherichia coli. Enkelytin shares sequence and structural similarities with the antibacterial C-terminal domain of diazepam-binding inhibitor. According to this similarity, a prediction of secondary structure is proposed for enkelytin and discussed in relationship to its biological activity.

Identification and characterization of N-glycosylated and phosphorylated variants of proenkephalin A-derived peptides in bovine adrenal medulla, spinal cord and ileum

Recent studies have shown that during its biosynthesis in bovine adrenal medulla, the opioid precursor proenkephalin A, may be both N-glycosylated and phosphorylated. To investigate whether these chemical modifications were common to proenkephalin A processing in other tissues, we have sought to characterize enkephalin-containing peptides from bovine adrenal medulla, spinal cord and ileum. The peptides were identified using antiserum L189, specific for the C-terminus of Met-enkephalin Arg6Gly7Leu8 (MERGL), and L152, specific for the C-terminus of Met-enkephalin Arg6Phe7 (MERF). Glycosylated MERGL-immunoreactive peptides of 23, 20, 16 and 13 kDa were identified in adrenal medulla using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and concanavalin A-Sepharose affinity chromatography. Sephadex G50 gel filtration fractionated the glycosylated peptides into two immunoreactive peaks. Similar peaks of concanavalin A-binding MERGL immunoreactivity were detected in extracts of spinal cord and ileum, although there were differences in relative proportions of the two peaks. Antiserum L152 identified phosphorylated N-terminally extended variants of MERF when boiling water extracts of adrenal medulla, spinal cord and ileum were separated by anion exchange chromatography. In adrenal medulla these peptides were more than 99% phosphorylated, whereas in both ileum and spinal cord there was a relatively higher proportion of the unphosphorylated peptide. The results indicate that N-glycosylation and phosphorylation of proenkephalin A occurs in adrenal medulla, spinal cord and ileum, although there are tissue-specific differences in the relative proportions of the modified and unmodified peptides.