Alkaline Phosphatase Isoenzymes

Alkaline phosphatase (ALP) in the circulation is a mixture of isoforms from the liver, kidney, bone and intestine. There are three genes for ALP - intestinal, placental and the liver/kidney/bone gene. The latter isoforms undergo post-translational modification with different carbohydrate sidechains being attached. In some instances it is important to be sure whether a raised plasma ALP is of liver or bone origin (e.g. malignancy, co-existing liver disease etc.). ALP isoenzymes can be resolved using isoelectric focusing into all the major forms.In particular marked elevations of plasma ALP can be seen in children (and occasionally in adults) associated with concurrent infections. This is caused by changes in the carbohydrate sidechain structure resulting in reduced recognition at clearance receptors and a prolonged half-life - transient hyperphosphatasaemia. This produces a characteristic pattern on isoelectric focusing with a particular response to neuraminidase treatment which aids recognition. It is a benign condition and the importance of identifying it is to avoid unnecessary investigations e.g. ERCP, bone scans etc.