Metabolic instability of myelin protein and proteolipid fractions.

Abstract

Turnover of protein components of myelin in vivo was shown to be comparable to that of other brain proteins. Myelin was purified, to remove soluble contamination, by water shock (fraction M2) and centrifugation on sucrose and CsCl gradients. These methods gave two fractions, one of which consisted of characteristic myelin membranes free of visible contamination (M3), and the second consisted of fragmented membranes and vesicles (M4).Incorporation following intracisternal injection of [14C]lysine occurred at later time periods in myelin compared to mitochondria, probably as a result of different rates for penetration of precursor. The half‐lives for the different categories of protein and proteolipid components prepared by Triton X‐100 acetate procedures were markedly heterogeneous. In myelin, the highest incorporation occurred in the soluble basic proteins with half‐lives of 14‐21 days (fraction III), followed by the interface (Folch‐Lees, fraction II) material with half‐lives of 12–24 days, and the insoluble proteins (Wolfgram, fraction III) with half‐lives of 28–32 days. The half‐lives varied with the pulse period used (15–360 min) and give evidence of both short‐lived and long‐lived proteins. Heterogeneity of turnover was supported by the complex disc‐gel patterns found with electrophoresis on acrylamide using a phenol‐urea solvent system for solubilization. In comparable mitochondrial fractions, incorporation was highest in the interface material (half‐life 6–4 days), followed by insoluble proteins (half‐life 3–6 days), and lowest in the soluble (brain) fraction (half‐life 6–26 days).Considerable hydrolase activity was observed in crude fractions and in hypotonic myelin fractions, particularly acid and neutral proteinases, aminopeptidase, and monoacyl and dipeptidyl arylamidases. It appears that hydrolases associated with the sheath could be involved in turnover, but in most cases are not associated with the highly purified membranes prepared in sucrose and CsCl gradients. Purification resulted in an increased specific activity of an aminopeptidase hydrolysing Leu‐Gly‐Gly as the substrate, and of ribonucleoside‐2′ : 3′‐cyclic‐phosphate esterase. It is concluded that these two hydrolases are intrinsic components of the myelin sheath membranes.