In my study, I evaluated the enzymatic activities of a formulation on soil enzymes—specifically phosphatase, dehydrogenase, and phytase.
For phosphatase, the activity was calculated in terms of p-nitrophenol (pNP) released per minute (mol⁻¹/min),
For dehydrogenase, it was based on TPF released per minute,
And similar substrate-based quantification was used for phytase.
However, in several related papers, the enzyme activity is reported in EU x 10⁻¹ (enzyme units) or sometimes in U/g or U/ml.
I'm trying to understand:
1. How can I convert my results (e.g., mol/min or μmol/min) into enzyme units (EU or U)?
2. Is there a standard definition or factor to use for this conversion?
3. What is the difference between using "EU" and "U"—are they interchangeable?
4. Can the final unit be expressed as U/g of soil or U/ml of extract?
Any guidance on standard practices or reference methods for such conversions in soil enzyme assays would be greatly appreciated!
Thankyou
"Units" of enzyme is a very specific term used primarily in the sale of commercial enzymes. The Unit is often defined by an initial researcher or manufacturer in terms of mass/time, moles/time, or absorbance change per time. If you ever do purchase enzymes commercially, make sure you read the definition of the unit as I have had manufacturers with definitions as disparate as grams / hour and mg / min.
I am not sure about soil science research , but in most biochemical journals you will want to cite your activity in either a mass / time or a moles / time value (a cursory Google search indicates that soil science use the same units). Most appropriate would be a turnover number (per second), but this requires a much more precise measurement of moles of active enzyme. Yours seems to be more of a large scale analysis of activity.
You state that there are other papers that use the Unit form. You may want to investigate what unit definition they use and perhaps cite that in your work.
To convert enzyme activity:
• pNP mol⁻¹/min = U (1 μmol/min = 1 U)
• U/g → EU/10⁻¹ = U/g × 0.1
• TPF/min → U/g: Use a standard curve to convert to μmol, then divide by sample mass
In short: convert to μmol/min, adjust for sample weight, and scale as nee
Enzymatic activity is commonly measured in terms of the amount of product formed per unit time. The most frequently used units include:
- μmol pNP/min (para-nitrophenol release)
- TPF/min (triphenyl formazan, often from dehydrogenase assays)
These can be converted into standard enzyme activity units such as:
- U/g: Enzyme units per gram of sample
- EU/10⁻¹: Enzyme units per 0.1 g of sample (often used in soil enzyme activity studies)
2. Definitions
1 Enzyme Unit (U) = The amount of enzyme that catalyzes the conversion of 1 μmol of substrate per minute under assay conditions.
3. Conversion Steps
Example:
Measured activity = 0.5 μmol pNP/min from 0.2 g dry sample
To Convert to U/g:
U/g = μmol/min ÷ sample weight in grams
= 0.5 ÷ 0.2 = 2.5 U/g
To Convert to EU/10⁻¹ g:
EU/10⁻¹ = (U ÷ sample weight) × 0.1
= (0.5 ÷ 0.2) × 0.1 = 0.25 EU/10⁻¹
4. If Activity is Measured in TPF/min
1. Convert TPF absorbance or mass to μmol using a standard curve.
2. Then use the same formulas as above to calculate U/g or EU/10⁻¹ g.
4. Note
Always ensure:
- Time is in minutes
- Product amount is in μmol
- Sample weight is in grams (g)
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