Lipase

EC 3.1.1.3

The enzyme lipase catalyzes the hydrolysis of triglycerides (triacylglycerols). Lipase is a glycoprotein with at least two known isoenzymes¹. Lipase is very non-specific, catalyzing the breakdown of most triglycerides. The reaction shown below is just one example of such a reaction catalyzed by lipase. In this case the substrate is triolein, a triglyceride in which all three fatty acids are oleic acid, a common substrate used in lipase reagents. Lipase preferentially cleaves the fatty acids on the ends, leaving a beta-monoglyceride as shown below. This monoglyceride spontaneously (but slowly) rearranges to an alpha-triglyceride which can then be cleaved by lipase². Thus eventually all three bonds will be cleaved by lipase resulting in three fatty acids and one molecule of free glycerol.

Lipase is somewhat unusual in that its common substrates (triglycerides) are not soluble in water and therefore must be emulsified in order to be hydrolyzed by lipase. This emulsification is performed in vivo by bile salts³. Lipase can only act on the surface of the dispersed substrate, and in addition requires a cofactor, colipase, which helps it attach to this surface^4,5.

The main clinical utility of lipase along with amylase, is for the diagnosis of acute pancreatitis. Although both are usually measured, lipase is far superior to amylase^6,7 as a marker for pancreatitis, reaching 95% sensitivity and specificity⁸. Lipase assay methods are extremely variable and may use emulsified triglycerides or water soluble analogs. Intra-assay variability is one of the main limitations of lipase’s clinical utility⁹. Nevertheless some have asked the question as to whether lipase should not replace amylase altogether, even with including amylase isoforms, for the diagnosis of acute pancreatitis¹⁰.

Other diagnostic applications of lipase have been occasionally reported including obstruction of the pancreatic duct and pancreatic cancer¹¹, but in general, the main utility of lipase is limited to acute pancreatitis.

REFERENCES

1. 1. De Caro, A., et. al. Human Pancreatic Lipase: A Glycoprotein. Biochimica et Biophysica Acta, 490, 411-419, 1977
2. 2. Burtis, C.A., Ashwood, E.R. and Bruns, E.R. Tietz Textbook of Clinical Chemistry, 4th Edition, W.B. Saunders Company, 2006.
3. 3. Borgstrom, B. The Action of Bile Salts and Other Detergents on Pancreatic Lipase and the Interaction with Colipase. Biochimica et Biophysica Acta, 488, 381-391, 1977.
4. 4. Semeriva, M. and Desnuelle, P. Pancreatic Lipase and Colipase. An Example of Heterogenous Biocatalysis. Adv Enzymol Relat Areas Mol Biol, 48, 319-370, 1979.
5. 5. Chapus, C. et. al. Minireview on Pancreatic Lipase and Colipase. Biochemie, 70, 1223-1234, 1988.
6. 6. Lott, J.A. and Lu, C.J. Lipase Isoforms and Amylase Isoenzymes: Assays and Applications in the Diagnosis of Acute Pancreatitis. Clinical Chemistry, 37, 361-368, 1991.
7. 7. Al-Bahrani, A.Z. and Ammori, B.J. Clinical Laboratory Assessment of Acute Pancreatitis. Clinica Chimica Acta, 362, 26-48, 2005.
8. 8. Keim, V. et. al. A Comparison of Lipase and Amylase in the Diagnosis of Acute Pancreatitis in Patients with Abdominal Pain. Pancreas, 16, 45-49, 1998
9. 9. Lessinger, J.M. and Ferard, G. Plasma Pancreatic Lipase Activity: From Analytical Specificity to Clinical Efficiency for the Diagnosis of Acute Pancreatitis. Eur J Clin Chem Clin Biochem, 32, 377-381, 1994.
10. 10. Smith, R.C., et. al. Should Serum Pancreatic Lipase Replace Serum Amylase as a Biomarker of Acute Pancreatitis? ANZ Journal of Surgery, 75, 399-405, 2005.
11. 11. Lombard, D., et. al. Is Bile Salt-Dependent Lipase Concentration in Serum of Any Help in Pancreatic Cancer Diagnosis? Pancreas, 8, 581-599, 1993.