Solubilization and characterization of glutamate binding sites from porcine brain

The majority of l-glutamate binding sites in the synaptic membranes isolated from porcine brain were solubilized by a mixture containing Triton X-100 and digitonin. The solubilized l-glutamate binding sites bind l-glutamate reversibly and with a K(d) value of 0.67 ?M. The solubilized sites also bind l-aspartate, cysteate, and homocysteate, but not N- methyl- d -aspartate , kainate or quisqualate. Various salts, sodium chloride, sodium acetate, potassium chloride, calcium chloride, and magnesium acetate, enhance the l-glutamate binding activity of the solubilized preparation. When solubilized preparations were fractionated by gel-filtration chromatography, no binding activity was detected in the resulting fractions, whereas activity could be partially recovered in the combinations of different fractions. l-Glutamate binding activity of the glutaraldehyde-treated solubilized preparation was eluted out of the gel-filtration column as a single peak. The results suggest that this l-glutamate binding site consists of at least two components and that phospholipids may play important roles in the receptor function.