Fabry disease: Diagnosis by α-galactosidase activities in tears

The enzymatic diagnosis of hemizygotes with Fabry disease and heterozygous carriers was accomplished by the fluorometric determination of α-galactosidase activities in tears. Two components of total α-galactosidase activity were differentiated by their relative thermostabilities and by chromatography on DEAE-cellulose. The major component, α-galactosidase A, was thermolabile and represented approximately 90% of total activity; the remaining activity was thermostable, eluted at a slightly higher salt concentration and was designated α-galactosidase B. A single, symmetric pH optimum was observed for total α-galactosidase activities from heterozygotes and normal individuals, whereas the total activity from hemizygotes, which was about 10% of that in normal controls, had a broad pH profile, identical to those for α-galactosidase B activities from all individuals studied. The apparent K_m values for total activities were 3.2, 4.0, and greater than 13 mM for normal individuals, heterozygotes, and hemizygotes, respectively. In contrast, apparent K_m values for α-galactosidase B activities were greater than 13 mM for all individuals, further suggesting that the residual activity in hemizygotes with Fabry disease represented the α-galactosidase B component. Of the potential inhibitors studied, α-d-melibiose was found to competitively inhibit total α-galactosidase activity (K_i ~ 10 mM). These studies demonstrate that tears provide an easily obtainable source of freshly secreted enzyme for the diagnosis of hemizygotes and heterozygotes with Fabry disease and suggest that tears may be useful for the diagnosis of other inborn errors of metabolism.