Leucine Aminopeptidase (LAP) is an intracellular enzyme that exists in the hepatobiliary system and, to a much smaller degree, in the pancreas and the small intestine. When disease or injury affects those organs, the cells lyse and LAP is spilled out into the bloodstream.
Produced almost exclusively by the liver, LAP is used in diagnosing liver disorders and in the differential diagnosis of increased levels of alkaline phosphatase (ALP). LAP levels tend to parallel ALP levels in hepatic disease. LAP is a sensitive indicator of cholestasis; however, unlike ALP, LAP remains normal in bone disease. LAP can be detected in both the blood and the urine. Patients with elevated serum LAP levels will show elevations in urine levels. When the urine LAP level is elevated, however, the blood level may have already returned to normal.
This enzyme was originally thought to be elevated during pregnancy and thus serve as an indication of the viability of the fetal-placental unit. However, it has been discovered that the enzyme found in pregnancy is immunochemically different from LAP.
Causes of False Leucine Aminopeptidase Indications
- Pregnancy may cause increased values if tested by the enzyme method. Although there is not a quantitative increase in this “LAP-like” enzyme, its activity is increased. This causes a false increase in the LAP if tested by the enzyme method.
- Drugs that may cause increased LAP levels include Estrogens and Progesterones.
Normal Leucine Aminopeptidase Levels
Male: 80-200 units/mL or 19.2-48.0 units/L (SI units)
Female: 75-185 units/mL or 18.0-44.4 units/L (SI units)
2-18 units/24 hr
Causes of High Leucine Aminopeptidase Levels
LAP is an enzyme that exists in the liver and biliary cells. Disease or injury of these tissues will cause the cells to lyse. LAP will spill out into the bloodstream, and levels will rise. This is the case of Hepatobiliary disease as a resul of:
- Hepatic Necrosis.
- Hepatic Ischemia.
- Hepatic Tumor.
- Hepatotoxic Drugs.