Further studies on the smooth-surfaced membranes isolated from rat brain

The light and heavy smooth-surfaced membranes (LSM and HSM), which had densities corresponding to 1.08 M and 1.28 M sucrose, respectively, were isolated from rat brain and some of their biochemical properties were investigated. Both LSM and HSM showed high Na+,K+-ATPase activity and, in particular, in HSM the activity was four times (21.55 mumol/mg protein/h) higher than that of the brain homogenate. High 2',3'-cyclic nucleotide 3'-phosphodiesterase activity (293.4 mumol/mg protein/h) was characteristic of LSM. 5'-Nucleotidase and acetylcholinesterase activities were also higher in LSM than in HSM. SDS-polyacrylamide gel electrophoresis showed that LSM and HSM had many protein component and that low molecular weight proteins such as proteolipid protein and basic protein were almost absent, in contrast with myelin and myelin-like membrane. GM1 ganglioside constituted the major class of total ganglioside in both LSM and HSM. These biochemical findings suggested that LSM is a membrane that has not previously been described, or a membrane fraction related to the oligodendroglial plasma membrane.