Persistently raised serum acid phosphatase activity in a patient with hypophosphatasia: electrophoretic and molecular weight characterisation as type 5.

Hypophosphatasia is a rare disorder characterised by low levels of serum alkaline phosphatase activity resulting in abnormal phosphorylated metabolites and varying skeletal abnormality. We have followed a patient with adult type hypophosphatasia for over ten years who has also shown a persistently elevated tartrate resistant acid phosphatase activity (TRAP) without any obvious cause. Characterisation of this TRAP by polyacrylamide gel electrophoresis (pH 4.0) showed migration to band 5 position. Molecular weight determination by FPLC and an estimate of the molecular weight by gradient gel electrophoresis gave a molecular weight of 29,000-43,600. This molecular weight makes it unlikely for this to be a IgG/TRAP complex persisting in the circulation. Paranitrophenylphosphate was the preferred substrate. This characterises the enzyme as type 5 acid phosphatase which is of the mononuclear/phagocyte type, possibility of osteoclastic origin, though the tissue source remains unknown.