Principle of Method:
Hydrolysis of radiolabelled ceramide (labelled in the fatty acid moiety) at acid pH is followed by measuring the radioactivity in the fatty acid released after solvent partitioning and thin-layer chromatography. The ceramide for the assay is prepared chemically in the laboratory by condensation of a [14C] fatty acid (palmitic acid in the current batch) with sphingosine.
Uses and Limitations of the Method:
Deficiency of acid ceramidase is the primary defect in Farber disease (lipogranulomatosis). It is a rare disorder and the SAS laboratory is one of only a few in the world offering the assay. Unusually, activity of acid ceramidase is higher in normal leucocytes than in fibroblasts, and the former cells are certainly the material of choice for making the diagnosis. Prenatal diagnosis of the disease is possible by assay of chorionic villi or amniotic cells, but the SAS laboratory does not offer this at present. Two other ceramidases, with neutral and alkaline pH optima, respectively, do not appear to be affected in Farber disease. There are no characteristic storage substances in urine that assist in making the diagnosis, but demonstration of excess ceramide in a biopsy of one of the subcutaneous nodules would provide strong evidence for the disease.
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.
THE LABORATORY RECOMMENDS USE OF A COURIER SERVICE OR ROYAL MAIL SPECIAL DELIVERY FOR SENDING ALL SPECIMENS TO THE LABORATORY.