The colorimetric determination of urinary lipase.

With the use of titrimetric (1)) manometric (2)) stalagmometric (3), and calorimetric (4, 5) techniques, the esterase and lipase activities of sera and many tissues have been measured by employing a wide variety of substrates (6). The vast amount of data that resulted from these studies contains numerous discrepancies in the behavior of these esterolytic enzymes, which are still unexplained. Two facts seem to be generally accepted. First, the ideal substrates for lipase appear to be either monoesters or glycerides of long chain fatty acids, whereas non-specific esterase acts optimally on simple esters of short chain carboxylic acids (7, 8). Nevertheless, extensive overlapping exists as demonstrated by the ability of esterase to hydrolyze naphthyl laurate and lipase to hydrolyze tributyrin. This fact is probably the main reason for the considerable confusion. Secondly, among numerous compounds which inhibit or accelerate esterolysis, sodium taurocholate is the one most suited for the differentiation of esterase from lipase because of its inhibitory action on esterase and its accelerating effect on lipase. By utilizing these two facts, a calorimetric method was developed for the determination of esterase and lipase in the serum of dog and man with p-naphthyl laurate as the substrate (9). After enzymatic hydrolysis, the free P-naphthol was converted to a colored azo dye by the addition of a diazonium salt. The insoluble pigment was extracted with ethyl acetate, and the color measured in a photoelectric calorimeter. The values obtained in the absence of sodium taurocholate were a measure of non-specific esterase activity. If lipase was present in the serum, the color density in the tube with taurocholate was greater, and the difference between the two represented the amount of lipase. In 50 normal human sera, lipase was not present in measurable quantities with the use of this method, whereas in certain pathological sera lipase was found (10). Admittedly, small amounts of lipase may be overlooked because the inhibition of esterase by the taurocholate may be almost equaled by its acceleration of a small amount of lipase, although the net effect,