Lysosomal sphingomyelinase

Callahan, J.W., Jones, C.S., Davidson, D.J. and Shankaran, P., 1983, The active site of lysosomal sphingomyelinase evidence for the involvement of hydrophobic and ionic groups./. Neurosci. Res. 10 151-161. 

Schissel, S.L, Keesler, G.A., Schuchman, E.H., Williams, K.J., and Tabas, I. The cellular trafficking and zinc dependence of secretory and lysosomal sphingomyelinase, two products of the acid sphingomyelinase gene, J Biol Chem, 273 (1998a) 18250-18259. 

ACAT or a decrease in ACAT activity itself. Because much of the cholesterol accumulating in the cells appears to be associated with lysosomes, it is tempting to speculate that defects in lysosomal cholesterol transport arise in advanced foam cells. In this context, macrophages exposed to oxidized LDL can internalize a substantial amount of cholesterol, but there is relatively little stimulation of ACAT-mediated cholesterol esterification [8]. According to one model, oxysterol-induced inhibition of lysosomal sphingomyelinase leads to accumulation of lysosomal sphingomyelin, which binds cholesterol and thus inhibits transport of the cholesterol out of lysosomes (M. Aviram, 1995). 

Oxysterols have diverse roles in cholesterol efflux, a critical topic in foam cell biology. On the one hand, cells incubated with 7-ketocholesterol and 25-hydroxycholesterol have decreased cholesterol efflux. Possible mechanisms include inhibition of membrane desorption of cholesterol or phospholipids or, as mentioned above, inhibition of lysosomal sphingomyelinase leading to lysosomal sequestration of cholesterol (M. Aviram, 1995). On the other hand, the conversion of cholesterol by macrophage sterol 27-hydroxylase to 27-hydroxycholesterol and 3[l-hydroxy-5-cholestenoic acid, which are efficiently effluxed from cells, has been proposed to promote sterol efflux from foam cells (1. Bjorkhem,... 

A new type of glucosylceramidase cofactor has recently been described by Vaccaro et al. (1985). This protein, which seems to be tightly associated with the enzyme, stimulates the hydrolysis of glucosylceramide but not that of the water-soluble substrate. Similarly two small proteins that stimulate specifically lysosomal sphingomyelinase were recently observed by Christomanou and Kleinschmidt (1985). In both cases it is, however, still too early to speculate on the role and significance of these proteins. 

Otterbach, B., and Stoffel, W., 1995, Acid sphingomyelinase-deficient mice mimic the neurovisceral form of human lysosomal storage disease (Niemann-Pick). Cell 81 1053-1061. 

Sphingomyelin, a ubiquitous component of cell membranes, especially neuronal membranes, is normally degraded within lysosomes by the enzyme sphingomyelinase. 

Acid sphingomyelinase is a lysosomal enzyme that catalyzes the breakdown of sphingomyelin to ceramide and phosphoryl-choline.A deficiency of this enzyme leads to lysosomal accumulation of sphingomyelin in patients with Niemann-Pick disease. Recent data indicate that correct intracellular targeting of acid sphingomyelinase to lysosomes is dependent on the mannose 6-phosphate-mediated pathway. Does this imply that the I-cell patient will present with Niemann-Pick symptoms Can I-cell disease be viewed as a constellation of many lysosomal storage diseases ... 

Niemann-Pick diseases. These are also classified as lysosomal storage diseases. There are two main distinct sub-families type A (NP-A) and type B (NP-B), both caused by defects in the acid sphingomyelinase gene. Further type C diseases are caused by defects in a gene involved in LDL-cholesterol homeostasis, identified as the NPCl gene. 

The lysosomal acid sphingomyelinase (aSMase) isoform is inhibited by L-camitine, a cofactor of acyl-coenzyme A transport in mitochondria (107, 108), and by phosphatidylinositol polyphosphates, which occur in plant, yeast, and mammalian cells (109). 

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