It's worth a try, but you will have to consider the problem of background absorbance from the medium, because the assay for LDH activity is to measure the absorbance decrease at 340 nm in the UV of NADH in the presence of pyruvate. The medium may contain substances that absorb or scatter strongly at this wavelength. You can centrifuge the medium to pellet particulates. That will help.

Hi Professor Adam, very helpful information, thank you so much. Actually UV is a control factor in my work, as we are studying the effect of UV on Diel vertical migration of Zooplankton and associated behaviour as grazing (sloppy feeding in my case). so as you said if I take off UV in my experimentation, the assay for LDH will be more precise? I means as no reaction about the absorbance during experiment in UV, results assay after the experiment more correct ?

The UV irradiation of the zooplankton during feeding is not related to the UV absorbance measurement of LDH activity, which is done using a spectrophotometer or absorbance plate reader. I was thinking that you would take the zooplankton growth medium in which the zooplankton had fed, centrifuge it to clarify it, take 1 ml of the supernatant, put it into a quartz cuvette, add the substrates for LDH (pyruvate and NADH), and measure the decrease in absorbance at 340 nm due to the action of LDH. The control would be a sample of the same medium that never had any zooplankton in it.

Cool, thanks so much M. Shapiro. 

BTW, do you know if it will be possible to analyse LDH from field sampling ? I will surely mesure it in my experimentation. however, I still heist if it worth to analyse filed simple for this enzyme. (I dont know if it degrades fast or not)