Concentrations of an activator protein for sphingolipid hydrolysis in liver and brain samples from patients with lysosomal storage diseases.

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Abstract
The hydrolysis of sphingolipids by lysosomal enzymes requires the presence of additional proteins, which have been called activator proteins. The number of activator proteins, their specificity, exact mechanism of action, and response to a storage process all remain to be determined. In this study, antibodies to an activator protein known to bind sphingolipids and activate the enzymatic hydrolysis of GM1 ganglioside and sulfatide were used to estimate the concentration of this activator protein in small samples of liver and brain from patients with lysosomal storage diseases. By using rocket immunoelectrophoresis, the concentration of cross-reacting material (CRM) was determined. Control livers had an average of 0.95 +/- 0.18 (mean +/- 1 SD) microgram CRM/mg protein in the extracts, and control brains had an average of 0.25 +/- 0.14 microgram CRM/mg protein. Extremely high levels of CRM were found in extracts of livers from patients with type 1 GM1 gangliosidosis (15.1 and 16.9), and type A Niemann-Pick disease (10.7). Extracts of brain samples revealed a large amount of CRM in type 1 GM1 gangliosidosis (14.8), Tay-Sachs disease (5.3 and 8.7), and Sandhoff disease (13.5). Significantly elevated CRM was also measured in brain samples from patients with type 2 GM1 gangliosidosis, type A Niemann-Pick disease, metachromatic leukodystrophy, and Krabbe disease. The highest levels are found in those genetic diseases where the lipids stored, primarily or secondarily to the genetic defect, bind to this activator protein. This activator protein may have an important function in regulating intralysosomal lipid catabolism, and changes in its concentration in certain genetic diseases may be the cause of clinical, biochemical, and pathological heterogeneity found in the patients.