Assuming that the designation U refers to the usual micromole/minute definition of a unit in enzymology (not always the case), then U/L means micromolar/minute (concentration of product/time), which is different from micromole/minute (moles of product/time).

However, another possibility is that the U/L label refers to the activity of a stock solution of ALP, i.e. one liter of the stock solution contains an amount of enzyme that will generate a certain number of micromoles of product per minute.

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You may find these links useful:- http://www.endmemo.com/medical/unitconvert/Alkaline_phosphatase.php and https://www.convertunits.com/from/micromoles+per+minute/to/micromole/minute