Lipase in serum--the elusive enzyme: an overview.

Lipase is a glycoprotein with 420-449 amino acid residues and a M(r) of 46,000-56,000 for pancreatic lipase and 32,000-39,000 for serum lipase. Lipase is present in the pancreas, intestines, and a variety of other tissues. The concentration gradient between pancreatic tissue and serum lipase is approximately 20,000-fold. Serine, as part of an Asp-His-Ser triad, is the nucleophilic residue essential for catalysis. Lipase differs from other esterases by the presence of a hydrophobic recognition site. The optimal pH is between 7.5 and 10.0, depending on the reaction condition; the pI for the various forms of the enzyme has been reported as 5.80 and 5.85; 6.4, 6.8, and 7.0; and 7.4 for a purified fraction. Several authors report the presence of two molecular forms in the pancreas and three electrophoretic bands with lipolytic activity. In normal serum two bands have been observed; in pancreatitis as many as four bands have been seen. Lipolytic activity may not always be due to lipase. Assays specific for lipase require a triglyceride as substrate as well as the presence of colipase (a water-soluble and heat-stable protein, essential for lipase action), a secondary bile salt, and Ca2+. The clinical sensitivity of all modern assays is high because of selection of a low decision limit; the clinical specificity varies greatly but can be improved by increasing the cutoff point. Lipase determinations in pancreatitis are superior to amylase determinations. The reasons for the great variability of reports regarding the clinical utility of lipase are discussed, and the clinical utility of lipase determinations is summarized.