A case of aspartate aminotransferase macroenzyme

Aspartate aminotransferase (AST) macroenzyme can result in elevated AST activity in patients with no disease. This case report describes a six-year-old boy who presented to his family doctor with a history of chronic constipation and lower back pain. Routine blood tests showed normal creatine kinase activity (CK), normal alanine aminotransferase activity (ALT) but raised AST. The patient was referred to a hospital paediatrician for further investigation of the abnormal AST and back pain. The raised AST was confirmed as the only biochemical abnormality. Further investigation with polyethylene glycol, followed by measurement of AST in the supernatant, showed undetectable enzyme activity. The sample was sent to a specialist laboratory where it was analysed by Sephacryl S300 gel filtration. This procedure confirmed the presence of a high molecular mass form of AST. AST macroenzyme should be considered as a cause of isolated AST increase, which may avoid further costly investigations.