Katal
SI derived unit · quantity: catalytic activity
01 · Definition
The katal is the youngest SI unit: it was adopted in 1999, fifty years after the newton and almost a century after the first attempts to measure enzymes. It is also the only unit introduced not by physicists but at the request of clinicians.
One katal is the conversion of one mole of substance per second. The unit describes not the amount of enzyme but the rate at which it works: two samples of equal mass may differ in activity several times over if one has partly denatured. The quantity is enormous for biochemistry — real values lie in micro- and nanokatals, and clinical assays use katals per litre. Before the katal, biochemists used «units» U: micromoles per minute, and the switch to SI took two decades, because every reference interval had to be rewritten.
One and the same enzyme yields different katals at different temperature, pH and substrate concentration. That is why an activity is always quoted with the conditions of measurement: 37 °C, pH 7.4, substrate in excess.
02 · Conversion
Enter an activity — the sheet will convert it
The unit U — a micromole per minute — remains the de facto standard in reagent catalogues and research articles. One U equals 16.67 nanokatals, an inconvenient number, and that alone has held back the transition for a quarter of a century.
An activity without stated conditions is a meaningless number. In clinical biochemistry the standard is 37 °C and physiological pH; at 25 °C the same enzyme yields half as many katals.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| kat | katal, the SI unit | 1·10⁻⁶ | industry, fermentation |
| mkat | millikatal | 0.001 | enzyme preparations |
| µkat | microkatal | 1 | clinical assays |
| nkat | nanokatal | 1000 | test tube, trace amounts |
| pkat | picokatal | 1 000 000 | the limit of sensitivity |
| µmol/s | micromole per second | 1 | the same as µkat |
| U | unit of activity, micromole per minute | 60 | test forms, reagents |
| mU | milliunit | 60 000 | weak activities |
| kU | kilounit | 0.06 | enzyme packaging |
| mol/min | a mole per minute | 6·10⁻⁵ | old protocols |
| kat | katal | 1·10⁻⁶ | the present-day unit |
The unit U survives in laboratories because switching to katals requires re-establishing reference intervals for every enzyme. Sixteen-and-a-bit nanokatals is an awkward figure for a form.
03 · Orders of magnitude
from a single enzyme molecule to an industrial reactorALT within norm — 0.67 µkat/l
04 · Measuring instruments
What katals are measured with
The product of the reaction absorbs light at its own wavelength: the rate at which the optical density rises gives the activity directly.
Ninety-six reactions at once: the instrument records the kinetics in every well and computes the activity from the accumulation of product.
The workhorse of a clinical laboratory: hundreds of samples an hour, thermostatting, and output straight in katals per litre — where the standard is followed.
If the reaction releases acid, the activity is measured by the titrant consumed: how much alkali went into holding the pH steady.
05 · Writing rules
kat in lower case — and always with the conditions of measurement
The symbol is written with three lower-case letters: kat, not Kat and not KAT. A prefix goes in front of it: µkat, nkat. An enzyme activity without temperature, pH and substrate is not comparable with other measurements, so protocols state them next to the number.
Test forms carry «U/L» with no explanation — those are units U per litre, not katals. The difference is a factor of 16.67 million, and confusion in reference intervals is dangerous.
06 · Neighbouring units
The katal is a mole per second, so it rests on the mole and the second directly. Its dimension coincides with the rate of reaction in chemical kinetics, and dividing by the litre gives activity concentration — the thing printed on a blood test.
The turnover number k(cat) is the activity divided by the number of enzyme molecules: for carbonic anhydrase it reaches a million per second. The katal counts the whole sample, the turnover number a single molecule.
07 · Historical section
What enzymes were measured with before the katal
A micromole of substrate per minute. The international standard of biochemistry, alive today in reagent catalogues.
A unit of its own for ligase: defined through phosphate exchange in ATP. One of dozens of «in-house» units that biochemistry has never fully abandoned.
Activity was expressed through the fraction of substrate broken down in a fixed time. Every laboratory had its own procedure, and the numbers were incomparable even between neighbouring institutes.
The International Pharmaceutical Federation set its own units for digestive enzymes in preparations. They still appear on the packaging of medicines.
The unit was introduced for medicine — and medicine barely took it up
The katal entered the SI at the direct request of the International Federation of Clinical Chemistry: doctors needed a way to compare results from laboratories in different countries. But the transition required re-establishing reference intervals for dozens of enzymes, reprogramming analysers and retraining staff, so most laboratories went on printing «U/L». Twenty-five years later the katal remains a unit obligatory in standards and rare on forms: the line «ALT 40 U/L» occurs a thousand times more often than «ALT 0.67 µkat/l».
Catalogue · the International System of Units
A passport for every quantity
Seven base units, twenty-two derived ones with their own names, and an archive of systems that have gone out of use. Each has its own sheet: definition, conversion, instruments, rules of writing. In 18 languages.