Enzymes as Indicators ofIntracellular Reaction Conditions
E. MULLER
With 1 Figure
Abstract
In contrast to numerous data concerning enzyme activity in vitro, little is
known in a quantitative sense about enzymes inside the cell. As an example of
enzyme kinetics in vivo the urea-urease-system of plant cells was investigated. The
enzyme located in the groundplasm shows kinetics similar to that in water solution: the Michaelis constant and the activation energy in vivo are of the same
magnitude as in I!itro. - Furthermore, a specific inhibition of urease by chloramphenicol takes place in vitro. This effect enables the free concentration of the
antibiotic in the groundplasm to be roughly estimated.
Since metabolism is mainly connected with the activity of enzymes, it
is important to obtain quantitative information on the behavior of enzymes
inside the living cell. The magnitudes of enzyme parameters in vivo and
their regulatory change not only provide us with information on the enzyme
molecule itself, but are concerned also with reaction conditions at the site
of the enzyme activity. Much is known about reactions of soluble enzymes
in vitro under strictly defined conditions. However, even in vitro it is
sometimes difficult to get comparable results, since many interfering factors
can appear.
Serious difficulties arise if enzyme parameters are to be investigated in
the living cell, in which many enzymes are bound to a structure. Their
activity in Vil)O can be followed through changes in the state of the prosthetic group (cf. [1)). Most of these enzymes are not active unless in
connection with a lipid structure. The consequences of the binding of an
enzyme to a structure are known incompletely [12]. Even if unbound to a
structure, enzymes in vit!O do not in general yield good information on
their kinetics [6,8,9].
Both the concentration and the state of the enzyme substrate at the site
of activity are vastly unknown; many substrates are transported by active
mechanisms and unevenly distributed among the cell compartments. However, in the plant tissue (Jyymphaea alba and Taraxacum officinale) [17] urea, the
substrate of urease, is passively equilibrated between the cell and the surrounding solution and, within certain limits, evenly distributed inside the
E. MULLER
With 1 Figure
Abstract
In contrast to numerous data concerning enzyme activity in vitro, little is
known in a quantitative sense about enzymes inside the cell. As an example of
enzyme kinetics in vivo the urea-urease-system of plant cells was investigated. The
enzyme located in the groundplasm shows kinetics similar to that in water solution: the Michaelis constant and the activation energy in vivo are of the same
magnitude as in I!itro. - Furthermore, a specific inhibition of urease by chloramphenicol takes place in vitro. This effect enables the free concentration of the
antibiotic in the groundplasm to be roughly estimated.
Since metabolism is mainly connected with the activity of enzymes, it
is important to obtain quantitative information on the behavior of enzymes
inside the living cell. The magnitudes of enzyme parameters in vivo and
their regulatory change not only provide us with information on the enzyme
molecule itself, but are concerned also with reaction conditions at the site
of the enzyme activity. Much is known about reactions of soluble enzymes
in vitro under strictly defined conditions. However, even in vitro it is
sometimes difficult to get comparable results, since many interfering factors
can appear.
Serious difficulties arise if enzyme parameters are to be investigated in
the living cell, in which many enzymes are bound to a structure. Their
activity in Vil)O can be followed through changes in the state of the prosthetic group (cf. [1)). Most of these enzymes are not active unless in
connection with a lipid structure. The consequences of the binding of an
enzyme to a structure are known incompletely [12]. Even if unbound to a
structure, enzymes in vit!O do not in general yield good information on
their kinetics [6,8,9].
Both the concentration and the state of the enzyme substrate at the site
of activity are vastly unknown; many substrates are transported by active
mechanisms and unevenly distributed among the cell compartments. However, in the plant tissue (Jyymphaea alba and Taraxacum officinale) [17] urea, the
substrate of urease, is passively equilibrated between the cell and the surrounding solution and, within certain limits, evenly distributed inside the