The second definition of enzyme activity, initially introduced to take into account the introduction of the SI unit, is the katal (kat). One katal is defined as the amount of enzyme that will raise the rate of reaction by 1.0 mol per second in an assay system under defined conditions. In most instances the use of nanokatals (nkat; the amount of enzyme required to raise the rate of reaction by 1.0 nmol/s under defined assay conditions) is more appropriate.
Upon completion of the experimental element of the practical, students should calculate the number of enzyme units (both in terms of U and nkat) present per milliliter of the enzyme solution provided. In both cases, the calculations are initiated by determining the number of nanomoles of inorganic phosphate produced under the assay conditions used, using the standard curve generated. This represents the number of nanomoles of Pi (product) produced by a total of 0.5 ml of the enzyme solution over a period of 15 min, under standard assay conditions. Calculation of the number of standard enzyme units (U) present per milliliter of enzyme is therefore achieved by dividing the value obtained by 1,000 (conversion of nanomoles to micromoles), further dividing by 15 (assay duration), and multiplying by 2 (as only 0.5 ml of the enzyme was used in the assay). Calculation of the number of nanokatal enzyme units present per milliliter of enzyme provided is achieved by dividing the value obtained from the standard curve by 900 (conversion of 15 min to seconds) and multiplying by 2 (as only 0.5 ml of enzyme was used in the assay)