ABSTRACT
The history of enzymes is closely connected with the
knowledge obtained in the fields of foodstuff chemistry
and technology (1,2). It started around 1526 with
Paracelsus’ studies of fermentation products. The
enzyme levels of a certain food are ascertained to
determine its degree of freshness (as in the case of
oxidative enzymes in vegetables), to detect particular
treatments, such as pasteurization (easily monitored in
milk by measuring the levels of phosphatase and
lactoperoxidase), or to see whether decay or microbial
contamination has started. Enzyme levels can be
measured easily by observing how they act on their
substrates, since most of these enzyme-catalyzed
reactions have an absolute specificity. The first satis-
factory mathematical analysis of the course of an
enzyme-catalyzed reaction was made by Michaelis and
Menten in 1913, who suggested that the rate of
transformation of a substrate (v) is a function, over a
certain range, of the substrate [S ] and enzyme [E ]
concentration:
v ¼ kcat½E½S KM þ ½S ð1Þ
where kcat and KM are constants. Assuming that this equation is valid for most enzyme-catalyzed reactions,
the enzyme concentration (or activity) is proportional
to the rate of appearance of a product or disappear-
ance of a substrate, and independent of substrate
concentration, only when [S ] KM for which the above expression may be reduced to:
v ¼ kcat½E ð2Þ For this reason, it is desirable to know the Km (Michaelis constant) of an enzyme for a particular substrate in a particular food. This can easily be calculated through the Lineweaver-Burk (doublereciprocal) plot of Eq. (1). The variability in the Km value makes it difficult to describe a universal method of analysis for a certain enzyme, since KM values vary greatly for the same enzyme, depending on the substrate used and the food analyzed. This means that if enzyme activity/concentration is to be accu-
rately estimated, prior knowledge of the KM value is necessary.
