Several examples of two-step sequential reactions exist where, because
of the poor equilibrium conversion by the first reaction, it is desir
able to conduct the two reactions simultaneously. In such a scheme, th
e product of the first reaction is continuously removed by the second
reaction, thus not allowing the first reaction to approach chemical eq
uilibrium. Therefore, the first reaction is allowed to proceed in the
desired direction at an appreciable rate. However, in many biochemical
applications where enzyme catalysts are involved, the enzyme's activi
ties are strong functions of pH. Where the pH optima of the first and
second reaction differ by three to four units, the above reaction sche
me would be difficult to implement, In these cases, the two reactions
can be separated by a thin permeable membrane across which the desired
pH gradient is maintained. In this article, it was shown, both by the
ory and experiment, that a thin, flat membrane of immobilized urease c
an accomplish this goal when one face of the membrane is exposed to th
e acidic bulk solution (pH(b) = 4.5) containing a small quantity of ur
ea (0.01 M). In this particular case, the ammonia that was produced in
the membrane consumed the incoming hydrogen ions and thus maintained
the desired pH gradient. Experimental results indicate that with suffi
cient urease loading, the face of the membrane opposite to the bulk so
lution could be maintained at a pH that would allow many enzymes to re
alize their maximum activities (almost-equal-to 7.5). It was also foun
d that this pH gradient could be maintained even in the presence of a
buffer, which greatly enhances the transport of protons into the membr
ane. (C) 1993 John Wiley & Sons, Inc.