Egress of free NeuAc from normal lysosome-rich granular fractions was assessed at NeuAc concentration of up to 221 pmol/hexosaminidase unit, achieved by exposure of growing fibroblasts to 40-125 mM N-acetylmannosamine for up to 7 days. The normal velocity of NeuAc egress increased with NeuAc loading and with temperature, exhibiting at Q10 of 2.4, characteristic of carrier-mediated transport. Fibroblasts cultured from five patients with infantile free sialic acid storage disease (ISSD) contained approximately 139 nmol of free NeuAc/mg of whole cell protein, or 100 times the normal level. Differential centrifugation, as well as density gradient analysis using 25% Percoll, showed that the stored NeuAc cosedimented with the lysosomal enzyme β-hexosaminidase. The velocity of appearance of free NeuAc outside ISSD granular fractions was negligible, even at initial loading levels of up to 3500 pmol/hexosaminidase unit. The lack of egress from ISSD granular fractions was found for both endogenous and N-acetylmannosamine-derived NeuAc. Fibroblasts from ISSD parents did not accumulate excess free NeuAc and did not display a velocity of NeuAc egress significantly different from normal. The defect in ISSD, like that in Salla disease, appears to be an impairment of carrier-mediated transport of free NeuAc across the lysosomal membrane. Clinical and biochemical differences between Salla disease and ISSD may reflect differences in the amount of residual NeuAc transport capacity.
|Original language||English (US)|
|Number of pages||7|
|Journal||Journal of Biological Chemistry|
|State||Published - 1989|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology