Enzyme activity and composition of myelin and subcellular fractions in the developing rat brain

Metabolic studies on myelin. Evidence for a precursor role of a myelin subfraction

Myelin prepared from brain tissue of the developing rat (15 days post partum) can be separated into several subfractions. These are (1) ;myelin-like' and ;purified myelin', by the technique of Davison and co-workers (Agrawal et al., 1970b) and (2) ;membrane fraction,' ;light myelin' and ;heavy myelin' by the discontinuous-sucrose-gradient procedure described in the present paper. ;Myelin-like' and ;membrane-fraction' subfractions appear to be similar in chemical properties, but different in detailed morphology by electron microscopy. Both fractions are related to myelin, on the basis of demonstrable myelin basic protein by polyacrylamide-gel electrophoresis in sodium dodecyl sulphate and the presence of a myelin-marker enzyme, 2':3'-cyclic nucleotide 3'-phosphohydrolase. These two fractions have a low lipid content (17% for ;myelin-like' and 40% for ;membrane-fraction' subfractions) compared with myelin (67-72%). No cerebroside was detected in these two fractions, whereas cerebrosides are a major component of myelin itself. The administration of [2,3-(3)H]tryptophan to young rats results in more rapid incorporation into proteins of the ;myelin-like' and ;membrane-fraction' subfractions when compared with incorporation into myelin. Data are presented which are consistent with a precursor-product relationship for conversion of ;myelin-like' and ;membrane-fraction' subfractions into myelin.

Output of (14C)adenine nucleotides and their derivatives from cerebral tissues. Tetrodotoxin-resistant and calcium ion-requiring components

1. Neocortical tissues, exposed briefly to [(14)C]adenine and containing over 98% of their (14)C as adenine nucleotides, when superfused with glucose-bicarbonate salines released about 0.1% of their (14)C content/min to the superfusate. 2. Addition of unlabelled adenosine to the superfusing fluid increased the (14)C output three- to four-fold; half-maximal increase was given by about 40mum-adenosine, and reasons are adduced for considering the activity of adenosine kinase to be a major factor in conditioning the (14)C output. Adenosine similarly increased the enhanced (14)C output caused by electrical excitation of the superfused tissue; it brought about only a small increase in tissue glycolysis. 3. Output of (14)C from the [(14)C]adenine-labelled tissues was increased when Ca(2+) was omitted from the superfusing fluids, but electrical stimulation did not then liberate more (14)C. Nevertheless, such tissues still responded to electrical stimulation by increased glycolysis, and their (14)C output again became susceptible to increase by electrical stimulation when Ca(2+) was restored. 4. The six-fold increase in tissue glycolysis caused by electrical excitation was almost completely inhibited by tetrodotoxin at 0.1mum and above, but this was associated with about 50% inhibition only in the output of (14)C from tissues preincubated with [(14)C]adenine. The (14)C-labelled compounds of which output was most inhibited by tetrodotoxin were adenosine, inosine and hypoxanthine whereas output in a nucleotide fraction was little affected.

Criteria for identifying a myelin-like fraction from developing brain

Multiple criteria must be used to relate a membrane fraction (;myelin-like') to myelin. These reported in this communication are electron microscopy, marker enzymes, sodium dodecyl sulphate-polyacrylamide-gel electrophoretic patterns and lipid composition.