Myelin Enzymes and Protein Metabolism
Turnover of myelin has attracted interest in relation to clinical and experimental diseases involving demyelination (7, 11). In adult rats, myelin represents about 12 % of total fresh weight, of which some 25 % is protein (14) and earlier concepts for its relative inertness are difficult to reconcile with the known halflife of under 20 days for 90 % of brain proteins (11). Turnover encompasses both synthesis and breakdown, linked together in a manner to preserve the function and integrity of tissues. Breakdown, like synthesis, is an orderly process involving a large number of hydrolases (proteinase and peptide hydrolases) acting in a sequential manner for the production of amino acids and peptides (Fig. 1). Earlier studies from this laboratory have shown the presence of proteinases in crude fractions containing myelin (7, 10) but there has been some question concerning the degree of contamination by enzymes from other particulates. All studies on turnover in myelin must consider 1) the purity and structural nature of isolated material, 2) the relative metabolism of different layers of the myelin sheath, 3) the role of the cytoplasmic inclusions or other discontinuities within the myelin sheath, 4) in studies on incorporation, questions related to penetrability of the precursor, 5) the role of Schwann or satellite cells, and 6) the age of the animal.
KeywordsSatellite Cell Protein Metabolism Myelin Sheath Neutral Proteinase Consecutive Time Point
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