To measure the lipase activity you can follow the method given by Winkler and Stuckmann 1979 as given in our publications of lipase.

As you are using p-NPB .. you can prepare the same in acetone or isoprpanol (10mM) and run the reaction in PBS 7.5pH.

The reaction conditions based upon the type of lipase you using for your study.

Your can follow the publication 

https://www.researchgate.net/profile/Ashok_Nadda/publications

Hope it will help you.

Good luck

According to me the following can be used: 

A. Sod phosphate buffer (0.05M, 8 pH): 90ml (add sod deoxycholate 207mg and gum acacia 100mg)

B. 30mg pNPP solution in 10 ml isopropanol (luke warm for disolve, don't boil)

Add the above 2 solutions (A+B). now its your lipase substrate. After incubation reaction can be stopped by adding triton X100 50microL on ice.

Hope it will help. All the best.

Gupta, N., P. Rathi and Gupta, R. 2002. Simplifiedpara-nitrophenyl palmitate assay for lipases and esterases. Anal Biochem. 311, 98-99.

-AKM

Thanks for your responses. I was able to do the assay as mentioned and my samples showed potent inhibitory activity. But the positive standard orlistat did not show any inhibition.

I prepared a orlistat  stock solution (2000 ppm) in DMSO. Is it because of DMSO or any other problem.