Sialidase activities of cultured human fibroblasts and the metabolism of GM3 ganglioside
Free sialic acid has been found in the cell-conditioned medium of human foreskin fibroblasts. It is proposed that the accumulation of extracellular sialic acid may result from the hydrolysis of GM3 ganglioside on the cell surface of these fibroblasts. Sialidase activities with GM3 ganglioside and sialyllactitol as substrates were demonstrated in cell-conditioned medium, and the levels of their activities correlated positively with cell density. The GM3 sialidase activity at pH 4.5 was 4.1 and 38 pmol/h/ml of medium at sparse and confluent densities, respectively; the corresponding activities with sialyllactitol as the substrate were 12 and 75 pmol/h/ml of medium (pH 4.5). The pH versus activity profiles with GM3 as the substrate suggested the presence of a second sialidase with an optimal activity at pH 6.5 in the conditioned medium of preconfluent cells. This activity was virtually absent in the medium of contact-inhibited cells and could not be assayed with sialyllactitol as the substrate. The turnover of cell surface GM3 was assessed by pulse labeling human foreskin fibroblasts with a radioactive precursor of sialic acid ((1-14C)N-acetylmannosamine) and a radioactive precursor of ceramide ((3,3-3H2)serine). During a chase period of 24 h turnover of the doubly labeled cellular GM3 was observed; there was a loss of about 35% of the 14C-labeled sialic acid without any measureable loss of 3H-labeled ceramide from GM3. We have speculated that the enzyme-catalyzed removal of sialic acid from the GM3 ganglioside on the extracellular aspect of the plasma membrane may be a necessary event involved in the modulation of cell growth.
- Research Organization:
- Michigan State Univ., East Lansing (USA)
- OSTI ID:
- 7154986
- Journal Information:
- J. Biol. Chem.; (United States), Vol. 263:14
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GANGLIOSIDES
METABOLISM
O-GLYCOSYL HYDROLASES
ENZYME ACTIVITY
SIALIC ACID
BIOSYNTHESIS
CARBON 14 COMPOUNDS
CELL CULTURES
FIBROBLASTS
HEXOSAMINES
MAN
PRECURSOR
SERINE
SKIN
TRACER TECHNIQUES
TRITIUM COMPOUNDS
AMINES
AMINO ACIDS
ANIMAL CELLS
ANIMALS
BODY
CARBOHYDRATES
CARBOXYLIC ACIDS
CONNECTIVE TISSUE CELLS
ENZYMES
GLYCOLIPIDS
GLYCOSYL HYDROLASES
HEXOSES
HYDROLASES
HYDROXY ACIDS
ISOTOPE APPLICATIONS
LABELLED COMPOUNDS
LIPIDS
MAMMALS
MONOSACCHARIDES
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANS
PRIMATES
SACCHARIDES
SOMATIC CELLS
SYNTHESIS
VERTEBRATES
550201* - Biochemistry- Tracer Techniques