Biosynthesis of gangliosides in tissues

Ganglioside biosynthesis in rat liver. Characterization of three sialyltransferases

Three sialyltransferase activities involved in ganglioside biosynthesis were studied in Golgi-enriched preparations of rat liver: the formation of GM3, GD3 and GD1a. The conditions for the quantitative assays of these enzymatic reactions were standardized and optimized, with Triton X-100 being used as detergent. The apparent Km values of each sialyltransferase for N-acetyl-2-(5'-cytidylyl)neuraminic acid (1.5 mM with GM3 synthase, 0.2 mM with GD3 synthase, and 0.5 mM with GD1a synthase) and the respective glycolipid substrates (0.08 mM for lactosylceramide, 0.1 mM for GM3, and 0.5 mM for GM1) were determined. Competition experiments showed that the three sialyltransferase activities are three individual catalytic entities. Moreover, evidence was found that product inhibition may play a role in the regulation of the activity of sialyltransferases.

Biosynthesis in vitro of gangliosides containing Gg- and Lc-cores

On the basis of our previous and present studies with embryonic chicken brain system, we have proposed stepwise biosynthesis of GD1a (Gg-series) and LD1 (Lc-series) gangliosides, starting from ceramide (Fig. 4). At least three different galactosyltransferases GalT-2 (UDP-Gal:Glc-Cer), GalT-3(UDP-Gal:GM2) and GalT-4(UDP-Gal:LcOse3-Cer) and three different sialyltransferases SAT-1(CMP-NeuAc:Lac-Cer), SAT-2(CMP-NeuAc:GM3) and SAT-3(CMP-NeuAc:nLcOse4 Cer) are involved in the biosynthesis in vitro of these gangliosides. All six of these glycosyltransferases have been solubilized using nonionic detergents. Two forms of glycolipid:galactosyltransferases (GalT-3 and GalT-4) have been separated by DEAE sepharose CL-6B chromatography from solubilized supernatant of 11- to 13-day-old embryonic chicken brain. Using microisoelectric focusing (pH gradient 3 to 8) the galactosyltransferases (GalT-3 and GalT-4) have been separated from SAT-3. Two beta-N-acetylglucosaminyltransferases (GlcNAcT-2(UDP-GlcNAc:nLcOse4Cer(beta 1-3] and GlcNAcT-3(UDP-GlcNAc:nLcOse4Cer(beta 1-6] have also been solubilized from mouse T-lymphoma, P-1798, using Triton CF-54. These enzymes are involved in the synthesis of Ii-core gangliosides and 3H-products have been characterized by methylation studies. Further separation of these two GlcNAcT's are in progress.

Chemical and morphological studies on garfish peripheral nerves

Gangliosides were extracted, separated by thin layer chromatography, and quantitated in three cranial nerves of the garfish (Lepisosteus osseus): the completely unmyelinated olfactory nerve (OLF), and two nerves composed of both myelinated and unmyelinated fibers, viz., the main trunk of the maxillary nerve (MAX) and a branch of the maxillary nerve (BR-MAX). Morphological studies on each of these nerves were done to verify that the OLF had been excised free of any contamination from the accompanying myelinated BR-MAX, to aid in the interpretation of the biochemical findings, and to clarify the nature of the OLF supporting cell. The chief chemical findings were (1) documentation of the presence of gangliosides in nerves previously thought not to contain them, (2) demonstration that gangliosides can be associated with unmyelinated nerves, (3) demonstration of a greater proportion of one simple ganglioside (G-6) in the OLF but greater proportions of two complex gangliosides (G-2 and G-0) in the MAX and BR-MAX, and (4) that either GM4 or a variant of the GM3 is present in OLF. The morphological findings with respect to the difficulty of ascribing only peripheral nervous system characteristics to the OLF supporting cell are discussed in relation to the ganglioside band chromatographing slightly ahead of GM4 in the OLF.