Isolation and initial characterization of myelin-like membrane fractions from the nerve cord of earthworms (Lumbricus terrestris L)

We report here the isolation of fractions enriched in components of the myelin-like membranes surrounding the giant axons of the earthworm.Lumbricus terrestris L. The composition and purity of the fractions have been assessed using SDS-protein electrophoresis, Western immunoblots, and electron microscopy. Preliminary enzyme assays indicated that the mitochondrial marker, succinate dehydrogenase, has a similar specific activity distribution in earthworm nerve cord and in mouse liver sedimentation velocity fractions, however, the distribution of the total units of activity among the fractions seems to indicate the existence of smaller mitochondria in earthworm nerve cord compared with mouse liver mitochondria. In earthworm nerve cord fractions, Na⁺/K⁺ ATPase and Ca²⁺/Mg²⁺ ATPase were found to be enriched exclusively in the fraction containing large plasma and myelin-like membranes, while in the mouse liver fractions, the total units of these two enzymes were found to be distributed broadly among fractions. 5-Nucleotidase activity in the earthworm nerve cord seemed to be restricted to the microsomal fractions (endomembrane network), with a very low activity associated with the large plasma and myelin-like membrane fraction. We have established the presence of keratins or prekeratins in the myelin-like membranes, probably in the form of tonofilaments. However, we could not show that the desmosome-like structures, characteristic of these membranes, are composed of those proteins described for vertebrate epithelial desmosomes.