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 [2,3-3H]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.
