Influence of colipase on the turbidimetric determination of pancreatic lipase catalytic activity.

The influence of colipase on the turbidimetric measurement of the catalytic activity of pure human pancreatic lipase (EC 3.1.1.3) and of sera from pancreatitis patients was studied. A deoxycholate-stabilized triolein emulsion served as substrate. It was found that the activity of the pure, colipase-free lipase is strongly inhibited by deoxycholate, and can be blocked completely if normal serum, pure human albumin, or the globulin fraction of normal serum is present. The inhibition by serum is competitive. This finding largely excludes the existence of a specific lipase inhibitor in human serum and explains the non-linear response of activity to the amount of serum added, a frequently observed problem with various turbidimetric lipase methods. A high molar excess of colipase (greater than 250-fold) completely abolishes the inhibition of lipase, irrespective of the inhibitory factor studied. Sera of pancreatitis patients, when measured turbidimetrically without addition of colipase, exhibit elevated lipase activity only if they contain colipase. However, the activity measured is not a function of the serum lipase concentration alone but of the molar ratio of colipase to lipase. Since this ratio varies considerably and is usually too low to ensure complete activation of lipase, erroneously low or even false negative results are obtained. For this reason it is strongly recommended that an excess of colipase is used in turbidimetric lipase assays. It therefore also appears important to study the influence of the serum colipase level on non-turbidimetric lipase methods.