An investigation into the glycolipid metabolism of alpha-N-acetylgalactosaminidase-deficient fibroblasts using native and artificial glycolipids
Deficient activity of lysosomal alpha-N-acetylgalactosaminidase represents a recently recognized lysosomal disorder whose neurologic manifestation in infancy is infantile neuroaxonal dystrophy. The lysosomal enzyme defect, inherited as an autosomal recessive trait, was first identified in the two brothers, GD and BD. Metabolic modification of glycolipids with terminal alpha-GalNAc was studied in fibroblasts from these patients. [Ceramide-3H]Forssman-glycosphingolipid (GSL), the fluorescent C6-NBD-lyso-Forssman-glycolipid (GL) and a 14C-labelled neoglycolipid containing the blood group A trisaccharide were synthesized and used as probes in degradation studies with cell homogenates and with cells in culture. Assays of each of these substrates with fibroblast homogenates of the patients demonstrated the profound deficiency of alpha-N-acetylgalactosaminidase activity compared with controls. Residual activities in the patients' fibroblast homogenates were detected with all glycolipid substrates; those amounted to 6.3 +/- 3.7% (BD) and 12.8 +/- 6.3% (GD) of the mean activity in controls for [3H]Forssman-GSL, and to 2.2 +/- 0.8% (BD) and 3.6 +/- 1.8% (GD) for C6-NBD-lyso-Forssman GL, respectively. alpha-N-Acetylgalactosaminidase deficiency in intact cells was confirmed by TLC analyses, which showed impaired glycolipid modification in cell extracts obtained following addition of [3H]Forssman GSL and C6-NBD-lyso-Forssman GL to the culture media of fibroblasts from the patients.