Synthesis and processing of α-galactosidase A in human fibroblasts. Evidence for different mutations in Fabry disease

The synthesis and processing of the human lysosomal enzyme α-galactosidase A was examined in normal and Fabry fibroblasts. In normal cells, α-galactosidase A was synthesized as an M(r) = 50,500 precursor, which contained phosphate groups in oligosaccharide chains cleavable by endoglucosaminidase H. The precursor was processed via ill-defined intermediates to a mature M(r) 46,000 form. Processing was complete within 3-7 days after synthesis. In the presence of NH₄Cl and in I-cell fibroblasts, the majority of newly synthesized α-galactosidase A was secreted as an M(r) = 52,000 form. For comparison, the processing and stability of α-galactosidase A were examined in fibroblasts from five unrelated patients with Fabry disease, which is caused by deficient α-galactosidase A activity. In one cell line, synthesis of immunologically cross-reacting polypeptides was not detectable. In another, the synthesis, processing, and stability of α-galactosidase A was indistinguishable from that in normal fibroblasts. In a third Fabry cell line, the mutation retarded the maturation of α-galactosidase A. Finally, in two cell lines, α-galactosidase A polypeptides were synthesized that were rapidly degraded following delivery to lysosomes. These results clearly indicate that Fabry disease comprises a heterogeneous group of mutations affecting synthesis, processing, and stability of α-galactosidase A.