I need to prepare standard curve using tributyrin as substrate for a lipase activity assay. I don't know I must use how much enzyme (mg or g). Can you help me?
The standard curve usually refers to the product (butyric acid), not the substrate. If you have butyric acid, you don't need the enzyme to make the standard curve. Just measure the signal at various concentrations of butyric acid.
Setting up the enzyme assay is another matter. Set up the assay with a range of enzyme concentrations (µg/ml or, preferably, nM) in order to find a suitable one. Make sure you measure the initial rate of the reaction for each tributyrin concentration. A suitable enzyme concentration will be one with which you can measure the initial rate of product formation over the range of tributyrin concentrations in your standard curve. If you plot the initial rate of product formation versus enzyme concentration, you should get a straight line. If you plot the initial rate of product formation versus substrate concentration, you should get a curve showing saturation. From this curve you can get Km and Vmax.
Just for the record, there is a good chapter in the book Pseudomonas Methods and Protocols:
https://link.springer.com/protocol/10.1007%2F978-1-4939-0473-0_12
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