Hi. Yasmina, if I remember correctly, pacreatin is a mixture of all sorts of homeopathic stuff from cows, pigs, etc. My advice is to throw it away and use real trypsin substrates like BAPNA. 

The principle of determining enzyme unit is quantity product of enzymatic reaction. The trypsin work at peptide to result pepton that around of 5 amino acid polymer under specific condition pH and temperature. Trypsin work in casein of milk at alkaline condition with sodium carbonate and the technique incubation of hydrolysis in water-bath to setting of temperature.

The product result determine for period of times to know quantity and relation between concentration the product  and times enzymatic reaction respectively plotted to graph. Slope of linear regression is used for determine unit enzyme (unit/ mL). IU can be known by conversion quantity value the product to micro-mole per minute. 

thank you for your answers, the problem is I didn't get a linear graph to determine the slope Ahmed Syauqi

Hi Yasmina,

were you able to get the linear graph, if so would you mind sharing how you modify or how to get it work? I am not able to get the linear curve, thanks