Is there a way to reliably test the quantity of lactase enzymes in a solution? For example, if I were to culture multiple bacterial species which produce lactase, is there a way to count the lactase enzymes? Adding lactose and testing for glucose would not work because lactase is reusable, being able to perform reaction after reaction without changing. I could also separate the lactase from all the other polypeptides and perform a Biuret test against controls. That raises the issue of isolating the lactase. Any alternative ideas are accepted.

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The typical way to measure enzyme concentration is via enzyme activity: if there's more of an enzyme, you'll see faster conversion of reactant to product.

So actually yes, adding a known amount of lactose and testing for glucose is a good way to do this. See for example this 1961 paper:

Jasewicz, L., & Wasserman, A. E. (1961). Quantitative determination of lactase. Journal of Dairy Science, 44(3), 393-400.

Fifteen minutes’ incubation at 37° C. is used for the enzymatic reaction on a pure lactose substrate. The lactase activity is related to the amount of glucose released by the hydrolysis, as determined by an enzymatic micro-method

I'm not certain what the best way to do this using modern laboratory methods might be, but in general measuring glucose concentration in a solution is quite a common thing to do.

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