Alkaline phosphatase (ALP)

Alkaline phosphatase (ALP) is a hydrolase enzyme that cleaves phosphate groups at alkaline pH and is measured in serum as a composite of isoforms originating primarily from the liver, bone, intestine, and placenta. Elevated ALP most commonly reflects hepatobiliary disease — cholestasis in particular — or accelerated bone turnover as seen in Paget's disease, bone metastases, and hyperparathyroidism; concurrent elevation of GGT (and/or 5'-nucleotidase) alongside ALP points to a hepatic rather than osseous origin, since these enzymes are not produced by osteoblasts. Low ALP can indicate hypophosphatasia, zinc or magnesium deficiency, or hypothyroidism. In longevity contexts, persistently elevated ALP has been associated with higher all-cause and cardiovascular mortality in epidemiological cohorts, independent of liver disease status.