We have to report the concentration in pg/ml, the ELISA kit provider did not reveal the specific activity of the enzyme. In that case how to convert the units?
ELISA determines the quantity of a protein, not the activity. The normal way to quantify the amount by ELISA is to prepare a standard curve, whereby several known amounts of the target protein are tested in the ELISA. In this case, that would be done by adding to the plate various solutions of purified beta-galactoside containing known concentrations of the protein.
It's possible that the mIU/mL readout of your ELISA refers to the quantity of enzyme rather than its activity. The International Unit (IU) is a vague term, and it need not be the same as an enzyme activity unit. You need to find out what an IU of beta-galactosidase means. If it does refer to activity, the conversion to pg/mL would requiring knowing the specific activity (µmol product formed/minute/mg protein) of the particular enzyme-containing sample tested in the ELISA. This would have to be measured with the enzyme activity assay used to define the International Unit. Without that information, the conversion is impossible. Of course, if you perform the enzyme activity assay, what was the purpose of the ELISA?
Thank you so much for the answer. I have used ELISA to determine beta-galactosidase in serum samples. I want to report it in pg/ml. I have asked the manufacturer to provide the specific activity of the enzyme, but they are reluctant to disclose it. So is there any other way?
As you are determining enzymatic activity of unpurified samples as serum , the results must be expressed as microMol.mL-1 . min-1 or as mIU. mL-1. Conversion of microMol.mL-1 . min-1 to mIU. mL-1 must be informed on ELISA kit. Enzymatic activity is expressed as reaction velocity of product formation or substrate comsumption. Therefore, it's not possible to know the enzyme quantitates of each sample. I recommend the Reference below with calculations of enzymatic activity.
Reference
Analytical Biochemistry 316, 11-14, 2003 A fluorimetric method for the determination of pepsin activity
Purchase a purified sample of the same beta-galactosidase that you are measuring with the ELISA, if possible, and use it in the ELISA to generate a standard curve based on its protein concentration rather than its activity.
Thank you for the suggestions Adam B Shapiro and Roseli A Gomes Roseli A Gomes
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