Pyridine also improves the absorbance. Is better than the alternatives sodium and ammonium hydroxide.

You can check here:

http://link.springer.com/article/10.1007%2FBF02636814

Thank you Andre.

if i didn't  use pyridine it will affect enzyme assay value? 

I never tested, I'm almost sure it will decrease your precision, maybe to a point that you can't trust the results anymore.

If you can't use pyridine you should run some standards to check the possibility of doing the assay without it.

can suggest me how could i Prepare oleic acid standard? in what range and quantity  oleic acid standard usally prepared?

Hello Raja, pyridine not only (as a weak base) raises pH, but also forms a soluble complex with cupric ions, thus preventing cupric hydroxide precipitation.

Best regards 

Enrico