SUBCELLULAR LOCALIZATION OF RAT BRAIN ESTERASES

Abstract

The localization and properties of soluble and bound esterases of subcellular fractions of rat brain have been investigated. Bound esterases were extracted with 1% Triton X-100 and separated by starch gel electrophoresis. By these means a molecular population of isoenzymes was demonstrable that was quantitatively different from the isoenzyme population of the watersoluble esterase activity. The highest specific activity for α-naphthyl acetate hydrolysis was contained in the microsomal fraction and could be extracted by Triton. In contradistinction to whole brain and to other subcellular fractions, microsomes contained a molecular population of esterase isozymes which was qualitatively distinct from that of water extracts in that the fast moving A-type esterases were absent. In addition, there was present a heavy concentration of slow movinig B-esterase. Acetylcholinesterase could also be extracted from this fraction by Triton, migrated with B-esterase and actively hydrolyzed αnaphthyl acetate. Combined electron microscopic and quantitative chemical analysis of the subcellular fractions suggested that some bound nonspecific esterase may be localized in sub-synaptic membranes. The pI₅₀ values for E 600 of the soluble and insoluble, Triton X-100-extracted and Triton X-100-insoluble esterases are, respectively, 5.3, 7.5, 7.5 and 7.4. It is noted that these results may be determined in part by the participation of acetylcholinesterase in hydrolysis of the substrate. Mitochondria are virtually devoid of esteratic activity. C-type esterase (CMB-activated, E 600-resistant) occurred in both the bound and soluble state. A-, B- and C-type esterases exist in both bound and soluble forms and their chemical properties appear to be independent of their site of localization. Histochemical studies indicate that the B-type esterase is localized predominantly to cytoplasm of neurons and to neuropil. A-esterase is localized to droplets (presumed lysosomes) of neurons and other cell types, e.g., pericytes. The histochemical localization of C-esterase could not be determined.