Metabolic Instability of Myelin Proteins

Abstract

Myelin basic protein, purified from bovine spinal cord, is cleaved by a purified bovine brain acid proteinase to yield three peptide fragments which were purified by gel filtration procedures combined with preparative gel electrophoresis. The purity of the fragments was established by end group analysis, amino acid analysis, polyacrylamide disc gel electrophoresis, and by their encephalitogenic activity. Chemical and biological characterization shows that one'peptide, (peptide 11), originates from the N-terminal end of the basic protein and contains residues 1-43; a second peptide, (peptide I), contains residues 44-89 and a third peptide (peptide III), originates from the C-terminal region with an N-terminal phenylalanine, residue 90, through arginine, residue 170, of the basic protein. The combined amino acid composition of the three fragments account for the amino acid content of the basic protein. The structure of the three peptides demonstrates that the purified brain acid proteinase selectively cleaves the two phenylalanine-phenylalanine linkages formed between residues 43-44 and 89-90 of the basic protein molecule. This study shows that cathepsin D could be the endogenous enzyme responsible for the breakdown of basic protein to form smaller encephalitogenic fragments, some of which are known to be present in crude tissue extracts.It was established in previous studies that most myelin protein components are subject to turnover [l]. Basic protein of myelin is of particular interest because it is known to induce an autoimmune disease associated with demyelination, experimental allergic encephalomyelitis. Previously, Einstein et af. [2] showed that basic protein can be cleaved by acid proteinase to form smaller encephalitogenic fragments, but the composition of these, and the nature of the susceptible bonds cleaved, have never been established. Indeed, early studies on the isolation and structure of basic protein were accompanied by autolysis during extraction, leading to the fortuitous isolation of polypeptide fragments, some of which were encephalitogenic in a particular species [3-51. Therefore, it is of some interest to establish the exact mechanism of basic protein breakdown using a known intracellular enzyme, since this may have some bearing on demyelinating processes in experimental disease and, in addition, provide some insight into the general mechanism of protein turnover in brain. A limited and specific degradation of basic protein could lead to the production of smaller (diffusible) peptides capable of interaction at the immunological sites related to autoimmune phenomena. The present work was designed to determine the nature of the peptide bonds split by the action of brain cathepsin D, as well as the structure and biological activity of the resultant peptides based on the known structure of bovine basic protein [6]. In other tissues, Coffey and de Duve [7] showed that catheptic activity of lysosomes appear to play a rate-limiting role in the pathways of protein breakdown. Thus, studies on the...

Purification and Characterisation of a Membrane‐Bound Substance‐P‐Degrading Enzyme from Human Brain

“…Although C-terminal deamidating enzymes are known to occur and may be responsible for inactivating thyrotropin-releasing hormone [12], there is no evidence that substance P is inactivated by deamidation [19,30]. More recent studies on a thyroliberin-deamidating enzyme purified from rat [50] and bovine [22] brain have shown that this enzyme can also cleave other neuropeptidades such as luliberin, angiotensin I and neurotensin on the carboxyl side of their proline residues, and hence is referred to as a 'postproline cleaving enzyme'.…”

“…These include both cytosolic [12-151 and membrane-bound [13,14,16-181 substance-Pdegrading enzyme activities. A cytosolic enzyme has been partially purified from rat brain and shown to release phenylalanine and leucine preferentially from substance P, suggesting cleavage at two or more internal peptide bonds (Gln6-Phe7 or Phe7-Phe' and Glyg-Leu") [19]. Akopyan and coworkers [20] purified a neutral endopeptidase from bovine hypothalamus which degraded luteinizing-hormone-releasing hormone (luliberin) and somatostatin and also cleaved substance P, probably between Gln6-Phe' and Phe7-Phe'.…”

Subcellular distribution of particle-bound neutral peptidases capable of hydrolyzing gonadoliberin, thyroliberin, enkephalin and substance P

“…Among these enzymes, the pyroglutamate aminopeptidase [17,181, the postproline-cleaving enzyme [19 -211, a nonchymotrypsin-like endopeptidase [22, 231, a cation-sensitive neutral endopeptidase [24,251, two other endopeptidases [26,271 and several aminopeptidases are present in the cytosol [28] and are thus excluded from the inactivation of extracellular peptides. Some of them, however, may be involved in the degradation after internalization of the peptide.…”

Cathepsin D—Many functions of one aspartic protease

“…General protein degradation and turnover Many (including ↓) [36] Activation and degradation of polypeptide hormones, FGF [37] chemokines, growth factors and their receptors Plasminogen [38] Prolactin [39,40] Endostatin [41] Osteocalcin [42] Thyroglobulin [43] Insulin [44] Glucagon [45] IL-1 [46] IGFBP [47] Androgen receptor [48] Parathyroid hormone [49] MIP-1 alpha [50] MIP-1 beta [50] SLC [50] Activation of enzymatic precursors Cathepsin B [51,52] Cathepsin L [51,53] Transglutaminase 1 [54] Brain antigen processing Amyloid beta A4 protein [55,56] Tau [57] Tubulin [58] Myelin basic protein [59] Mhtt [60] Apolipoprotein E [61] Alpha-synuclein [62] Degradation of cytoskeletal proteins Neurofilaments [63] Actin [64,65] Myosin [64,66] Tropomyosin [65] Monocyte-mediated fibrinolysis Fibrinogen [67,68] Fibrin [67,68] Regulation of apoptosis Bid [69] Thiredoxin-1 [70] Processing of enzyme activators and inhibitors Prosaposin …”