1. 14C from [1−14C]glucose injected intraperitoneally into mice is incorporated into glutamate, aspartate and glutamine in the brain to a much greater extent than 14C from [2−14C]glucose. This difference for [1−14C]glucose and [2−14C]glucose increases with time. The amount of 14C in C-1 of glutamate increases steadily with time with both precursors. It is suggested that a large part of the glutamate and aspartate pools in brain are in close contact with intermediates of a fast-turning tricarboxylic acid cycle. 2. 14C from [1−14C]acetate and [2−14C]acetate is incorporated to a much larger extent into glutamine than into glutamate. An examination of the time-course of 14C incorporated into glutamine and glutamate reveals that glutamine is not formed from the glutamate pool, labelled extensively by glucose, but from a small glutamate pool. This small glutamate pool is not derived from an intermediate of a fast-turning tricarboxylic acid cycle. 3. It is proposed that two different tricarboxylic acid cycles exist in brain.

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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 M,) and centrifugation on sucrose and CsCl gradients. These methods gave two fractions, one of which consisted of characteristic myelin membranes free of visible contamination (MJ, and the second consisted of fragmented membranes and vesicles (M4).Incorporation following intracisternal injection of [14C]lysine occurred a t 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. I n myelin, the highest incorporation occurred in the soluble basic proteins with half-lives of 14-21 days (fraction 111), followed by the interface (Folch-Lees, fraction 11) material with half-lives of 12 -24 days, and the insoluble proteins (Wolfgram, fractionIII) with half-lives of 28 -32 days. The half-lives varied with the pulse periods 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. I n comparable mitochondria1 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.Alteration in the composition of myelin during growth, or in disease, is accompanied by severe malfunction of the nervous system. Despite considerable recent interest in myelin, the nature of the protein components and the manner they are complexed with lipids (proteolipids) within the sheath are poorly documented (for review see [4]). Previous studies on protein metabolism were handicapped by inadequate procedures for preparation of myelin free of contamination. The present study was aimed a t the investigation of such processes, with particular emphasis on comparison of turnover and breakdown of proteins in crude preparations and of those purified by hypotonic shock and sucrose and CsCl gradients.There have been several suggestions in the literature that breakdown...