SI UNITS in biochemistry

SI Units International units

SI UNITS in biochemistry

International units

The International System of Units (or Systems Internationale, SI) is now adapted by nearly all the international scientific bodies. The system is ac. claimed as being "logical and coherent". It provides a uniform scientific language for weights and measures. It is fundamentally divided into several units:

The litre (L) as a unit of volume remains more popular than the given SI unit of cubic metre (m). The litre is exactly equal to 1 cubic decimetre (dm)., that is, 1000 litre = 1 m³.

Multiples and submultiples of the litre are used for all measurements of volume in a clinical chemistry laboratory

The same rules apply to SI units for mass which is the kilogram (kg) and working unit is gram (g).

Weight, unlike mass, is measured in Newtons and is not to be confused with mass. The amount of substance in SI unit is the mole (mol). It means that the old unit of gram molecule, gram equivalent, etc. is now replaced by the mole. Similarly, molecular and equivalent weights in mg (mM, mEq), ug (UM), or ng (NM) is now expressed as mmol.umol. or mol respectively as shown below:

Molecular weight in g/L is expressed as M. mol/L, mol/L

Mol. Wt. in mg/L is expressed as mM, umol/L, uMole/L

Mol. wt. in ug/L is expressed as um, umol/L, Mole/L

Valency Normality (N) = Eq. Wt. (g)/L

Molality = Mol/kg = Mol. wt. in g/kg

In recent years, SI units are being widely used in reporting results of laboratory determinations. Results which were formerly expressed as mg/100 ml are now reported as mmol/L. This conversion can be easily done by using the formula:

For example to convert 90 mg/100 ml of glucose Molecular weight of glucose = 180

To convert mmol/L to mg/100 ml, the formula is

 International units

It is difficult to determine the molecular weight of enzymes. Earlier, each unit of enzyme activity for various enzymes was defined separately. To bring about uniformity in method of reporting, it is now recommended that all enzyme activities should be expressed in terms of an international unit (U). International unit is defined as the amount of enzyme activity which brings about the consumption of 1 micromole of substrate or formation of I micromole of product per minute under defined conditions.

This international unit is now changed to an SI unit, the katal. One katal of enzyme activity causes a change in concentration of substrate or product of one mole per second. Being a large unit, measurements are made in nkat/L.

I International Unit/L = 16.67 nkat/L

This SI unit is not commonly used.