Principle of Method:
Hydrolysis of the synthetic substrate 4-methylumbelliferyl-a-D-galactopyranoside at acid pH is followed by measuring the fluorescence of the liberated 4-methylumbelliferone after stopping the reaction with alkaline buffer. In the assay for blood cells, cultured cells and chorionic villi N-acetylgalactosamine is included in the reaction to inhibit a-galactosidase B (actually an a-N-acetylgalactosaminidase) which interferes with the specific determination of the A enzyme. In serum and plasma activity of a-galactosidase B is very low and the inhibitor is not required.
Uses and Limitations of Method:
Deficiency of a-galactosidase A is the primary defect in Fabry disease (also called Anderson-Fabry disease or angiokeratoma corporis diffusum). The phenotype is often sufficiently distinctive to suggest the diagnosis, and requests to test for the disease are usually specific, and not part of a white cell screen. If the test is normal, other lysosomal disorders to consider for patients with angiokeratoma are sialidosis, Schindler disease, b-mannosidosis, fucosidosis and aspartylglycosaminuria. Diagnosis of affected males can be made by showing a marked deficiency of the enzyme in serum or plasma, but it is advisable to confirm the result using white cells. Female heterozygotes, who sometimes show symptoms of the disease, cannot reliably be diagnosed using serum or plasma, but a partial deficiency for some of them is observed in white cells. The most useful test for heterozygotes is study of hair roots, whereby two populations of cells are tested for, one with normal and one with deficient activity, but even here results may not be conclusive. Results are expressed as a ratio a-galactosidase: a-hexosaminidase.
Specimen Requirements:
Blood: 5 ml lithium heparin (orange capped tube) unseparated and unfrozen. Send at room temperature to arrive at the laboratory within 24h of venepuncture.
For female heterozygotes hair roots may be needed. A sample of between 30 and 40 hair roots is required from each patient. Each root is placed in an Eppendorf tube covered with buffer and frozen at -70° C until analysed. Patients may come to our department (by appointment) for this test, or the roots can be sent to the laboratory on dry ice. Full details, and a bottle of buffer, are available on request.
THE LABORATORY RECOMMENDS USE OF A COURIER SERVICE OR ROYAL MAIL SPECIAL DELIVERY FOR SENDING ALL SPECIMENS TO THE LABORATORY.