The sialic acid residue of exogenous GM1 ganglioside is recycled for biosynthesis of sialoglycoconjugates in rat liver.
In order to assess metabolic recycling of sialic acid, GM1 ganglioside [nomenclature of Svennerholm (1964) J. Lipid. Res. 5, 145-155; IUPAC-IUB Recommendations (1977) Lipids 12, 455-468], 14C-radiolabelled at the acetyl group of sialic acid, was intravenously injected into Wistar rats, and the presence of radioactive sialic acid in liver sialoglycolipids (gangliosides) and sialoglycoproteins was ascertained. A time-course study (20 min-72 h) showed that the radioactivity present in the liver distributed in the following fractions, with reciprocal proportion varying with time: the protein (glycoprotein) fraction, the ganglioside fraction and the diffusible fraction, which contained low-Mr compounds, including sialic acid. Ganglioside-linked radioactivity gradually decreased with time; protein-linked radioactivity appeared soon after injection (20 min), reached a maximum around 20 h, then slowly diminished; diffusible radioactivity provided a sharp peak at 4 h, then rapidly decreased till disappearing after 40 h. The behaviour of bound radioactivity in the individual liver gangliosides was as follows: (a) rapid diminution with time in GM1, although with a lower rate at the longer times after injection; (b) early appearance (20 min) with a peak at 1 h, followed by continuous diminution, in GM2; (c) early appearance (20 min), peak at 1 h, diminution till 4 h, followed by a plateau, in GM3; (d) appearance at 60 min, maximum around 40 h and slow diminution thereafter, in GD1a, GD1b and GT1b. A detailed study, accomplished at 40 h after injection, demonstrated that almost all radioactivity present in the protein fraction was released by mild acid treatment and recovered in purified sialic acid; most of radioactive glycoprotein-bound sialic acid was releasable by sialidase action. In addition, the radioactivity present in the different gangliosides was exclusively carried by sialic acid and present in both sialidase-resistant and sialidase-labile residues. Only in the case of GD1a was the specific radioactivity of sialidase-resistant sialic acid superior to that of sialidase-releasable sialic acid. The results obtained lead to the following conclusions: (a) radioactive GM3 and GM2 were produced by degradation of GM1 taken up; GM3 originated partly by a process of neosynthesis; (b) radioactive GM1 consisted in part of residual exogenous GM1 and in part of a neosynthetized product; (c) radioactive GD1a originated in part by direct sialylation of GM1 taken up and in part by a neosynthetic process; (d) radioactive GD1b and GT1b resulted only from neosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)
Department of Medical Chemistry and Biochemistry, University of Milan, Italy.
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Ghidoni
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Trinchera
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Sonnino
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Chigorno
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Tettamanti
D000818:Animals; D002250:Carbon Radioisotopes; D002855:Chromatography, Thin Layer; D005677:G(M1) Ganglioside / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D008954:Models, Biological; D019158:N-Acetylneuraminic Acid; D009439:Neuraminidase; D051381:Rats; D011919:Rats, Inbred Strains; D012794:Sialic Acids / Q000378:metabolism*; D012795:Sialoglycoproteins / Q000096:biosynthesis*
D002250:Carbon Radioisotopes; D012794:Sialic Acids; D012795:Sialoglycoproteins; D005677:G(M1) Ganglioside; D009439:Neuraminidase; D019158:N-Acetylneuraminic Acid