STUDIES ON THE TURNOVER OF MOUSE BRAIN SYNAPTOSOMAL PROTEINS1

Abstract

Abstract—(l) The half‐lives of the proteins of various fractions of whole mouse brain increase with increasing insolubility; the supernatant and hypotonic‐extractable proteins had half‐lives of about 13 days, whereas the membrane proteins solubilized with Triton X‐100 and SLS had half‐lives of about 18 days. The proteins of the subfractions of synaptosomes had half‐lives ranging from 15 to 19 days; those in the cytoplasm had a half‐life of 18·3 days, in the membranes, about 17 days and in the synaptic vesicles, 15·6 days.(2) Although the half‐life of the synaptic vesicles was not significantly different from that of other synaptosomal subfractions, the vesicles exhibited a different protein pattern on acrylamide gels, an observation which implies that the proteins of the vesicles are qualitatively different from those of other synaptic membranes.(3) The uptake of labelled lysine into the cytoplasm of the synaptosomes of youg mice in vivo was very rapid.(4) The data derived from the relative specific radioactivities of synaptosomal fractions compared with their whole brain analogs support the contention that a sizeable fraction of the synaptosomal cytoplasmic protein was transported to the synapse by axoplasmic flow. The relative specific radioactivities of synaptosomal membrane and synaptic vesicle proteins rose much more quickly than the comparable activities for the cytoplasmic material, and the alternate possibility of synthesis in situ is discussed.