Salla disease is a lysosomal storage disorder characterized by
mental retardation and disturbed
sialic acid metabolism. To study endogenous synthesis and breakdown of
sialic acid, fibroblasts were incubated for 5 d in the presence and then in the absence of N-[3H]acetylmannosamine. Labeling of free
sialic acid was 5-10 times higher in mutant than in normal cells. Radioactivity decreased in 4 d by 75% in normal but only by 30% in mutant fibroblasts. The labeling pattern was not normalized upon coculture of mutant and normal cells. To study the metabolism of extracellular
sialic acid,
low-density lipoprotein (
LDL) was labeled in the
sialic acid moiety (periodate-NaB3H4) or in the
protein moiety (125I). Binding, internalization, lysosomal degradation, and exit of products of
protein catabolism were similar in normal and mutant fibroblasts. Upon incubation with
LDL labeled in the
sialic acid moiety, mutant cells accumulated 2-3 times more free
sialic acid radioactivity than normal fibroblasts, mostly in the lysosomal fraction. After a 24-h chase incubation, radioactivity in free
sialic acid decreased by 70-80% in normal but only by 10-30% in mutant cells. In mutant fibroblasts, 40% of the radioactivity remained in lysosomes, whereas no labeled free
sialic acid was detected in lysosomes from normal fibroblasts. We conclude that in
Salla disease, fibroblast endogenous synthesis of
sialic acid and lysosomal cleavage of exogenous
glycoconjugates is normal, but free
sialic acid cannot leave the lysosome. These findings suggest that the basic defect in
Salla disease is deficient transport of free
sialic acid through the lysosomal membrane.