Lysosome lipid storage disorder in NCTR-BALB/c mice: Spleen and lung lysosomes store unesterified cholesterol but differ in their phospholipid composition

A strain derived from a colony of BALB/c mice at the National Center for To xicological Research, Jefferson, AR, USA (NCTR-BALB/c) suffers from an auto somal recessive disorder characterized by proliferation of secondary lysoso mes with accumulation of unesterified cholesterol in several tissues. The u nesterified cholesterol content of spleens and lungs from the affected mice were elevated 8- and 3-fold respectively over age- and sex-matched control s. Postnuclear supernatants of tissue homogenates were fractionated by sucr ose density gradient centrifugation and the fractions were analyzed for une sterified cholesterol, protein and marker enzyme activities for lysosomes ( N-acetyl-b-D-glucosaminidase, beta -D-glucuronidase), plasma membrane (alka line phosphodiesterase I), endoplasmic reticulum (glucose-6-phosphatase) an d mitochondria (cytochrome oxidase). The enzyme distribution profile showed that lysosomes of affected tissues floated at low density regions (density 1.05-1.08) of the gradient and contained substantial amount of tissue unes terified cholesterol. These low density lysosomes were purified about 17-fo ld (58% yield) from spleen and about 6-fold (32% yield) from lungs with min imal contamination by other organelles They were mostly intact as judged by high latency for N-acetyl-beta -D-glucosaminidase activity (70-100%). Lyso somes of control tissues were not found at the low density regions. The dis tribution profiles for other organelles were similar between affected and c ontrol tissues. Phospholipid composition of low density lysosomes were dist inctly different from their respective tissue homogenates. Spleen and lung lysosomes were enriched in sphingomyelin and phosphatidylcholine respective ly. The results suggest that these lysosomes acquire their low densities du e to accumulation of unesterified cholesterol, the retention of which may b e aided by sphingomyelin and phosphatidylcholine content of the lysosomes.