(14C)Glucosamine incorporation into specific products in the nerve ending fraction in vivo and in vitro

The products ofin vivo andin vitro incorporation of radioactive glucosamine into glycoproteins associated with the nerve ending (synaptosome) fraction were studied by polyacrylamide gel electrophoresis and also by gel filtration of glycopeptides derived from these glycoproteins by pronase digestion.Afterin vivo labeling for 1 h a heterogeneous group of [¹⁴C]glucosamine labeled glycoproteins was found associated with the nerve ending fraction. The predominant [¹⁴C]glucosamine containing glycopeptide derived by pronase digestion of the nerve ending fraction had an apparent molecular weight of 1250. It constituted a larger percentage of the labeled glycopeptides associated with the nerve ending fraction, as compared with other subcellular fractions. After 4 h ofin vivo labeling other glycopeptides constituted a greater percentage of the total labeled glycopeptides derived from the nerve ending fraction. This suggests that a different group of [¹⁴C]glucosamine containing glycoproteins were now relatively abundant in nerve endings. Presumably they had arrived by axoplasmic transport to the nerve endings.Afterin vitro labeling the nerve ending fraction incorporated [¹⁴C]glucosamine more actively than the microsomal or mitochondrial fractions. The radioactive products found upon polyacrylamide gel electrophoresis of the nerve ending fraction appeared to be less heterogeneous than afterin vivo labeling. Upon pronase digestion a single class of labeled glycopeptides was found with the same apparent molecular weight as the predominant labeled glycopeptide in the nerve ending fraction afterin vivo labeling.The results indicate that some [¹⁴C]glucosamine is incorporated into specific products by components of the nerve ending fraction and that this glycosylation cannot readily be attributed to contamination with other particles containing glycosyl transferases. Other glucosamine containing products are apparently transported to the nerve endings after glycosylation in the nerve cell body.