Functional compartments of sulphatide metabolism in cultured living cells: evidence for the involvement of a novel sulphatide-degrading pathway.
The modes of uptake and degradation of radiolabelled cerebroside sulphate (CS or sulphatide) were investigated in cultured living skin fibroblasts and Epstein-Barr virus-transformed lymphoblastoid cell lines established from control individuals and patients affected with metachromatic leucodystrophy (cerebroside sulphatase deficiency), multiple sulphatase deficiency and low-density-lipoprotein-receptor-negative familial hypercholesterolaemia. In both cell types, CS was taken up through a non-receptor-mediated process. In fibroblasts, CS degradation occurred intralysosomally as was evident from the findings that fibroblasts from metachromatic leucodystrophic patients accumulated the sulphatide and that chloroquine inhibited its degradation by normal cells. In contrast, under similar conditions of CS availability, lymphoblastoid cell lines from patients with metachromatic leucodystrophy could degrade the incorporated sulphatide exactly as their normal counterparts. This metabolic pathway was also fully active in lymphoblastoid cells from patients with multiple sulphatase deficiency and was not inhibited by chloroquine treatment. These data are consistent with a non-lysosomal type of hydrolysis. In addition to the lysosomal and non-lysosomal compartments, a third compartment was identified in the two cell types which is probably formed by the pool of the sulphatide molecules incorporated into the plasma membrane. This is the first report on the existence of a CS-degrading pathway in intact cells with deficient lysosomal cerebroside sulphatase activity.