Biomedical Engineering Reference
In-Depth Information
FIGURE 12.21
Basic design of the switchable enzyme reactor.
provide a novel type of switchable reactor. In the remaining parts of this section, switchable enzyme
reactors through hybridizing receptor function and enzymatic catalysis on a lipid-bilayer membrane
are described [120-123]. The basic design of the switchable enzyme reactor is summarized in
Figure 12.21. An artifi cial receptor having amine function and enzymes (lactate dehydrogenase
[LDH]) was immobilized on the surface of the lipid bilayer. Copper ion (Cu
2
+
) was used to con-
trol the activity of LDH, because Cu
2
+
is known to inhibit LDH activity. The switching mecha-
nism of the enzyme reactor by chemical signal is illustrated in Figure 12.22. The LDH (effector)
immobilized on the bilayer vesicle is inhibited by the metal ion in the initial state (OFF state).
When a suitable signal molecule (aldehyde compound in this case) is added to the system, a signal-
receptor complex (Schiff's base) is formed through a specifi c reaction. Because the signal-receptor
complex has a higher affi nity for the metal ion than the enzyme, the metal ion is removed from the
enzyme and is activated (ON state). The activity of the immobilized LDH was monitored through
the pyruvate reduction effi ciency. The relative activity of the pyruvate reduction in the presence
of Cu
2
+
, compared with that under a Cu
2
+
-free condition, was drastically decreased. The addition
of a single component of a signal molecule or a receptor did not essentially change this situation.
However, the coexistence of the signal and the receptor drastically increased the LDH activity.
This result indicates that the formation of the Schiff's base suppressed the enzymatic inhibition by
removing Cu
2
+
. This can be regarded as a conversion from molecular recognition to an enzymatic
reaction (catalytic amplifi cation).
The used receptor contains azobenzene moiety that is capable of photoisomerization. Therefore,
the LDH activity can be controlled by photo-signal irradiation (Figure 12.23). A large spectral
change upon the alternate irradiation of UV and visible lights was reproducibly and repeatedly
observed at 290-390 nm, indicating that the receptor tends to be a
cis
-isomer and a
trans
-isomer
upon UV and visible light irradiations, respectively, and that the isomerization can be freely repeated.
