3. This value is too high for photometric determination; rather an absorption decrease within the range of 0.1/min will be feasible. To achieve this about 0.016 IU of LDH should be added to a single assay. Preparing a stock solution of lactate dehydrogenase with just 1 IU/ml and adding 0.02 ml from it to 0.98 ml of the assay mixture,

the absorption decrease per min will be 0.126, just within the expected range. In comparison, 1 kat lactate dehydrogenase produces an absorption change of 6,300,000/s. Since one second is too short for measuring, the absorption decrease within 1 min would be 378,000,000, far away from any reality. To obtain an absorption decrease of 0.1/min, 0.00000000026 kat lactate dehydrogenase is needed. A common lactate MK8776 dehydrogenase preparation contains about 500 IU/mg protein, 1 IU–2 µg. 1 kat=60,000,000 IU, corresponding to 120 kg SCH727965 ic50 lactate dehydrogenase, a completely unrealistic quantity. Obviously calculation with katal is somewhat difficult. However, the problem can be avoided by using nanokatal (nkat) for calculation, 1 nkat=0.06 IU, 1 IU=16.67 nkat. There are

also enzyme units in use that differ from both definitions with respect to the time unit (e.g. 1 h) and the amount of substrate. As far as possible such units should be adapted to katal or IU to enable comparison with other reports. This is in principle possible with respect to the time unit, but it is not always easy to define accurately the substrate concentration, e.g. with enzymes degrading macromolecules

like proteins or starch. Such substrates vary in their molecular mass and, in the strict sense, not either the macromolecule itself but the binding to be cleaved is the real substrate. Correspondingly the Anson units for proteases are defined according to the colour intensity of the assay instead of a molarity (Peterson, 1979). Enzyme units serve to quantify the amount of an enzyme. The amount of the enzyme is not defined by its mass (protein) rather by its function. This is reasonable, because the catalytic potential and not the protein is the essential feature of the enzyme. Even enzymes comparable in their purity can differ considerably in their activities; a partially inactivated enzyme cannot be discriminated from an active one only by protein analysis. The purity of an enzyme is usually expressed by the specific enzyme activity, i.e. the enzyme units divided by the protein content of the respective enzyme preparation. The higher the value the purer the enzyme, lower values indicate either impurities or partial inactivation of the enzyme. Enzyme units can serve to evaluate the amount of enzyme required for a distinct enzyme assay. As already mentioned, for theoretical reasons the enzyme concentration should be as low as possible, the detection limit determining the lowest amount.