F abry disease is an X-linked disorder of glycosphingolipid metabolism caused by a deficiency of alpha-galactosidase A (alpha-Gal A). We identified a novel mutation of alpha-Gal A gene in a family with Fabry disease, which converted a tyrosine at codon 365 to a stop and resulted in a truncation of the carboxy (C) terminus by 65 amino acid (AA) residues. In a heterozygote of this family, although the mutant and normal alleles were equally transcribed in cultured fibroblasts, lymphocyte alpha-Gal A activity was approximately 30% of the normal control and severe clinical symptoms were apparent. COS-1 cells transfected with this mutant cDNA showed a complete loss of its enzymatic activity. Furthermore, those cotransfected with mutant and wildtype cDNAs showed a lower alpha-Gal A activity than those with wild type alone (approximately 30% of wild type alone), which suggested the dominant negative effect of this mutation and implied the importance of the C terminus for its activity. Thus, we generated mutant cDNAs with various deletion of the C terminus, and analyzed. Unexpectedly, alpha-Gal A activity was enhanced by up to sixfold compared with wild-type when from 2 to 10 AA residues were deleted. In contrast, deletion of 12 or more AA acid residues resulted in a complete loss of enzyme activity. Our data suggest that the C-terminal region of alpha-Gal A plays an important role in the regulation of its enzyme activity.