Abstract
Abstract Golgi apparatus fractions from rat liver contained all glycosyltransferases which catalyze the in vitro biosynthesis of N-acetylneuraminylgalactosylglucosylceramide (Gm3), (N-acetylneuraminyl)2-galactosylglucosylceramide (Gd3), N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (Gm2), galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosyglucosylceramide (Gm1), and N-acetylneuraminylgalactosyl-N-acetylgalactosaminyl-(N-acetylneuraminyl)-galactosylglucosylceramide (Gd1a) from the corresponding precursors. Relative to total particulate fractions, these transferases were enriched 22 to 27 times in Golgi apparatus. Rough endoplasmic reticulum fractions also contained a full complement of these glycolipid glycosyltransferases; specific activities with endoplasmic reticulum were 2 to 4 times those of total particulate fractions. Plasma membrane fractions displayed negligible glycolipid glycosytransferase activities. The combined Golgi apparatus and endoplasmic reticulum fractions accounted for more than 80% of the total homogenate glycolipid glycosyltransferase activities. The results show that gangliosides are not synthesized at the surface membrane. As with membrane glycoproteins, gangliosides appear to be glycosylated in endoplasmic reticulum and Golgi apparatus during transport to the surface membrane.
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