Further characterization of two forms of N-acetyl-alpha-galactosaminidase from human liver

Multiple exo-glycosidases in human serum as detected with the substrate DNP-α-GalNAc. I. A new assay for lysosomal α-N-acetylgalactosaminidase

This paper presents a new assay to determine the activity of the lysosomal enzyme α-N-acetylgalactosaminidase (Naga, EC 3.2.1.49) in human serum. It is based on the use of a new chromogenic substrate, DNP-α-GalNAc (2,4-dinitrophenyl-N-acetyl-α-D-galactosaminide) and is performed at pH 4.3 and 37 °C. This allows continuous monitoring of the absorbance of the released DNP. The assay can be performed with a standard spectrophotometer. Compared to established methods using an endpoint assay with MU-α-GalNAc (4-methylumbelliferyl-GalNAc), the present method gives a ca. 3-fold higher specific activity, while only one tenth of the serum concentration in the assay is required. Hence, the assay is at least 30-fold more sensitive than that with MU-α-GalNAc. The pH dependence of the reaction with DNP-α-GalNAc in the pH 3.5 to 6.5 region, while using 4% serum in the assay, shows only one peak around pH 4. This pH optimum is similar to that reported with MU-α-GalNAc. In the accompanying paper (S.P.J Albracht and J. Van Pelt (2017) Multiple exo-glycosidases in human serum as detected with the substrate DNP-α-GalNAc. II. Three α-N-acetylgalactosaminidase-like activities in the pH 5 to 8 region. BBA Clin. 8 (2017) 90-96), the method is used to show that, under special assay conditions, three more Naga-like activities can be uncovered in human serum.

alpha-N-acetylgalactosaminidase deficiency with mild clinical manifestations and difficult biochemical diagnosis

Two additional patients with alpha-N-acetylgalactosaminidase (alpha-NAGA) deficiency are described. An 11-month-old girl with nonconsanguineous parents had generalized seizures and no angiokeratoma. Biochemical investigation showed persistent slight oligosacchariduria; enzymatic analysis of plasma, leukocytes, and fibroblasts revealed profound alpha-NAGA deficiency. Heterozygote enzyme levels were found in both parents. The mother has epilepsy, and epilepsy is present in the father's family. A younger, clinically healthy brother also had the enzyme deficiency. Electron microscopy of lymphocytes from the index patient showed no vacuolization. Incubation of cultured fibroblasts with Helix pomatia lectin showed the presence of intracellular N-acetylgalactosamine-containing storage material, not present in a series of 12 normal fibroblast lines. Our cases cannot be classified definitely as infantile cases. Biochemically the diagnosis could easily have been missed. Urinary oligosaccharide pattern after resorcinol staining was identical to those previously described, but excretion was significantly lower than in the reported infantile cases and the bands disappeared after the urine was desalted. The enzyme defect in leukocytes would have been missed with one of the commercial substrates used. For this mild variant of alpha-NAGA deficiency, the clinical pattern is not yet clear; a longer follow-up period is needed.

Biosynthesis of human alpha-N-acetylgalactosaminidase: defective phosphorylation and maturation in infantile alpha-NAGA deficiency

The biosynthesis of human alpha-N-acetylgalactosaminidase (alpha-NAGA) was studied in normal fibroblasts and in cells from patients with infantile alpha-NAGA deficiency. Normal alpha-NAGA is synthesized as a 52 kDa precursor which matures to a 49 kDa species through phosphorylation and carbohydrate triming. Fibroblasts from the patients synthesize normal amounts of a 52 kDa precursor, however phosphorylation does not occur and this precursor is subsequently degraded intracellularly.

alpha-N-acetylgalactosaminidase deficiency, a new lysosomal storage disorder

A new lysosomal storage disease with autosomal recessive inheritance is described in two male siblings of 5 1/2 and 4 years of age. Clinical manifestations started after 9 months of age with neurological symptoms, followed by progressive psychomotor deterioration. Urinary oligosaccharide excretion was abnormal and showed a characteristic pattern on chromatography. Enzyme assays showed a profound deficiency of lysosomal alpha-N-acetylgalactosaminidase in cultured fibroblasts, leukocytes and plasma from the patients and reduced activity in material from the parents. The deficiency was demonstrated both with an artificial substrate and a natural one, the blood group A trisaccharide. Excessive intra-lysosomal storage of alpha-N-acetylgalactosamine-containing material was demonstrated in cultured fibroblasts from the patients, using the lectin from Helix pomatia which is specific for terminal alpha-N-acetylgalactosamine residues.

Morphological and biochemical changes in muscle and peripheral nerve in Fabry's disease

In a case of Fabry's disease, microscopic, ultrastructural, and biochemical studies of a muscle biopsy were performed, as well as microscopic, ultrastructural, and morphometric studies of a nerve biopsy. Pleomorphic lipid inclusions were observed in muscle fibers, fibroblasts, and endomysial capillaries. Moreover, the thermolabile isoenzyme A of alpha-D-galactosidase was almost completely absent. In the nerve specimen, polymorphous lysosomes were noted in perineural cells, in fibroblasts, and in endothelial and perithelial cells in association with some nonspecific degenerative changes. The morphometric data revealed a loss of large myelinated fibers, an uncommon finding in Fabry's disease, and a decrease of the average diameter of the unmyelinated fibers, which was related to axonal sprouting. The relationship between the pain attacks and the increased number of the small unmyelinated fibers is discussed.