Enzyme activity is typically measured in units of micromoles of substrate converted per minute.
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Reference literature and enzyme assay guidelines report that the standard enzyme unit (U) is defined as the amount of enzyme that catalyzes the conversion of one micromole of substrate per minute.
The nutritional value of a diet and its bioavailability in fish depend on three primary capacities: (a) ingestion, (b) digestion, and (c) absorption. Among these, digestive capacity, defined here as the total enzyme activity available to hydrolyze the bonds of dietary macromolecules to obtain hydrolysis products that are ultimately converted into absorbable micromolecular units, establishes the upper limit for the bioaccessibility of nutrients. To clarify usage and measurement, we conducted a systematic SCOPUS survey (January 2020–June 2024; 62 relevant articles). Most studies either omit a clear definition of digestive capacity or conflate it with digestive organ morphology or isolated enzyme activities. We compared indicators and assay conditions (substrate type, pH, temperature, and expression of units), revealing significant inter-study variability. Based on this synthesis, we propose four operational definitions: (a) Extract Theoretical Volume (ETV)—calculated volume of extract, considering both the solvent volume (SV) used for tissue homogenization and the tissue’s water content; (b) digestive capacity (U)—the total catalytic activity present in the digestive tract at the moment of sampling, where 1 U is the amount of enzyme catalyzing the formation of 1 µmol of product per minute under species-specific physiological pH, ionic strength, and temperature, with the total activity expressed as U fish−1, U organ−1, or U g−1 fish or U g−1 organ, enabling direct comparisons across studies; (c) Digestive Processing (DP)—the total number of bonds hydrolyzed during a given digestion time, whether instantaneous or over a defined period; and (d) Digestive Processing Index (DPI, U-min or U-h), which integrates digestive capacity over time. This framework provides a harmonized checklist for assay standardization and advances comparative studies in fish digestive physiology.
enzyme unit (U) = 1 μmol min−1 (micromole per minute). 1 U corresponds to 16.67 nanokatals. Enzyme activity as given in katal generally refers to that of the
Enzyme assays are laboratory methods for measuring enzymatic activity. They are vital for the study of enzyme kinetics and enzyme inhibition.
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V
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= Reaction volume
The SI unit is the katal, 1 katal = 1 mol s−1 (mole per second), but this is an excessively large unit. A more practical and commonly used value is enzyme unit (U) = 1 μmol min−1 (micromole per minute). 1 U corresponds to 16.67 nanokatals.
Enzyme activity as given in katal generally refers to that of the assumed natural target substrate of the enzyme. Enzyme activity can also be given as that of certain standardized substrates, such as gelatin, then measured in gelatin digesting units (GDU), or milk proteins, then measured in milk clotting units (MCU). The units GDU and MCU are based on how fast one gram of the enzyme will digest gelatin or milk proteins, respectively. 1 GDU approximately equals 1.5 MCU.
An increased amount of substrate will increase the rate of reaction with enzymes, however once past a certain point, the rate of reaction will level out because the amount of active sites available has stayed constant.
Initial rate experiments. When an enzyme is mixed with a large excess of the substrate, the enzyme-substrate intermediate builds up in a fast initial transient. Then the reaction achieves a steady-state kinetics in which enzyme substrate intermediates remains approximately constant over time and the reaction rate changes relatively slowly. Rates are measured for a short period after the attainment of the quasi-steady state, typically by monitoring the accumulation of product with time. Because the measurements are carried out for a very short period and because of the large excess of substrate, the approximation that the amount of free substrate is approximately equal to the amount of the initial substrate can be made. The initial rate experiment is the simplest to perform and analyze, being relatively free from complications such as back-reaction and enzyme degradation. It is therefore by far the most commonly used type of experiment in enzyme kinetics.
Progress curve experiments. In these experiments, the kinetic parameters are determined from expressions for the species concentrations as a function of time. The concentration of the substrate or product is recorded in…
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