Thermal Biosensors in Biotechnology - Advances in Biochemical Engineering Biotechnology - page 48

Table 6. ET as toxing guard system

Analyte Immobilized Concentration Reference

compound [mmol/l]

amines monoamine oxidase 0.001-0.7 Svensson et al.1979

azide Saccharomyces cerevisiae 10-100 mg/l Hundeck 1991

Cu 2+ urease 0.1 Mattiasson et al.1977b

cyanide rhodanase + injectase 0.02-1 Mattiasson et al.1977a

Saccharomyces cerevisiae 10-100 mg/l Hundeck 1991

peroxidase 20-100 ppm Fischer 1989

Saccharomyces cerevisiae 1-20 mg/l

2,4-DNP Hundeck 1991

Hg 2+ urease 0.01 Mattiasson et al.1977b

Saccharomyces cerevisiae 10-100 mg/l Hundeck 1991

insecticides acetyl choline esterase <0.0034 Mattiasson et al.1979

organophosphate hydrolytic enzymes >0.03 Mattiasson et al.1979

sulfide peroxidase 10-1000 ppm Fischer 1989

Zn 2+ alkaline phospatase 0.01-1 Satoh 1991a

inhibited by special analytes are shown.Due to reversibility problems,inhibition
of enzymes is a difficult procedure (Schmidt et al.1995).Thus,microorganisms
would be a better choice.

3.6
Enzymatic Amplification

Efficient working of ET depends on enzymatic reactions accompanied by high
reaction enthalpies.A reaction with low enthalpy is often amplified by a sequen-
tial or cycling system.In a sequential system,a product ofthe first or subsequent
reaction is converted by a coimmobilized enzyme with a high enthalpy effect.
For instance,several oxidases produce hydrogen peroxide.Due to low reaction
enthalpies of oxidases, coimmobilized catalase effects an amplification in a
sequential step (

H = -100.7 kJ/mol). Another example is arginase analysis.
Here,an enzyme column with coimmobilized arginase and urease is employed.
Arginase cleaves arginine to ornithine and urea whereas urease converts the
latter with a high enthalpy effect to ammonia and carbon dioxide. A column
without urease gives much lower thermometric signals.
The enzymatic cycling systems are characterized by a combination of enzy-
mes (mostly oxidases and dehydrogenases) that are capable of a multiple rege-
neration of a substrate.Here,a repeated regeneration of the substrate causes an
accumulation of heat and amplifies the signals.Enzymatic cycling systems have
an important requisite,namely the analyte concentration has to be well below its
Michaelis-Menten-value K M .Otherwise,the reaction speed is not proportional
to the analyte concentration (Bergmeyer,1983).
The combination ofcycling systems gives very high amplification factors.The
latter describes the ratio of amplified and non-amplified signal in the linear

D