Glutamyl and aspartyl moieties of cerebral proteins: enrichment in membrane-containing microsomal subfractions.

Abstract

Abstract— The total mixed proteins (excluding proteolipids) were isolated from various tissues of the cat and from subcellular fractions of cat cerebral cortex and were compared for contents of total glutamyl and total aspartyl residues and total amide N. The proteins from renal cortex, testis and diaphragm were more acidic (proportionally more glutamyl and aspartyl residues with no increase of amide) than those from cerebral cortex, subcortical white matter and liver. Also the proteins isolated from cerebral cortical microsomes were more acidic than those from highly pure nuclear, mitochondrial and soluble subcellular fractions of cerebral cortex. Hepatic microsomal proteins gave very similar analyses.Subfractionation of the microsomal preparations from cerebral cortex and liver into deoxycholate‐insoluble (ribosomal) and deoxycholate‐soluble (‘membrane’) fractions localized the acidic proteins to the latter, ‘membrane’ subtraction. The ribosomal proteins isolated from cerebral cortex (and liver) had an amino acid composition very similar to that reported previously for ribosomal proteins in a number of tissues from various species. Upon column chromatography on DEAE‐celluIose, the deoxycholate‐soluble (‘membrane’) subtraction of hepatic microsomes yielded a peak (eluted with 0‐2 M‐phosphate, pH 5‐6, plus 0‐5 M‐NaCl) that contained an exceptionally high proportion (30 per cent) of glutamyl and aspartyl residues. We suggest that such acidic proteins may be characteristic of membranes and may subserve important metabolic functions therein.