Enzyme Specific Activity Calculator
Enzyme units from an absorbance rate (ΔA/min, ε, path length, volumes) and specific activity in U/mg — with fold purification and yield across purification steps.
How it works
rate (M/min) = ΔA/min ÷ (ε·l); U = µmol/min = rate × assay volume; U/mL = U ÷ mL enzyme added; specific activity = U/mL ÷ mg/mL
One enzyme unit converts one micromole of substrate per minute under defined conditions. Beer–Lambert turns a spectrophotometer slope into a molar rate, the assay volume turns it into an amount, and dividing by the enzyme volume and protein concentration gives the two numbers every purification table needs: U/mL and U/mg. Specific activity rises as contaminating protein is removed, and the ratio to the crude extract is the fold purification.
Worked example
ΔA340 = 0.045/min with NADH (ε 6,220): 7.2 µM/min in a 1 mL assay = 7.2 nmol/min = 0.0072 U from 20 µL, so 0.36 U/mL; at 2.5 mg/mL that is 0.145 U/mg.
Frequently asked questions
What if the absorbance decreases?
Use the magnitude of the slope — NADH oxidation, for instance, is followed as a fall at 340 nm.
How do I convert to katal?
1 U = 16.67 nkat (1 µmol/min = 1/60 µmol/s).
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For teaching and planning — verify against your lab protocol and instrument documentation.