Enzyme Technology
Medical applications of enzymes
Development of medical applications for enzymes have been
at least as extensive as those for industrial applications, reflecting the
magnitude of the potential rewards: for example, pancreatic enzymes have been in
use since the nineteenth century for the treatment of digestive disorders. The
variety of enzymes and their potential therapeutic applications are
considerable. A selection of those enzymes which have realised this potential to
become important therapeutic agents is shown in Table 4.4. At present, the most
successful applications are extracellular: purely topical uses, the removal c
toxic substances and the treatment of life-threatening disorders within the
blood circulation.
Table 4.4 Some important therapeutic enzymes
Enzyme
|
EC number
|
Reaction
|
Use
|
Asparaginase
|
3.5.1.1
|
L-Asparagine H2O
L-aspartate + NH3
|
Leukaemia
|
Collagenase
|
3.4.24.3
|
Collagen hydrolysis
|
Skin ulcers
|
Glutaminase
|
3.5.1.2
|
L-Glutamine H2O
L-glutamate +
NH3
|
Leukaemia
|
Hyaluronidase a
|
3.2.1.35
|
Hyaluronate hydrolysis
|
Heart attack
|
Lysozyme
|
3.2.1.17
|
Bacterial cell wall hydrolysis
|
Antibiotic
|
Rhodanase b
|
2.8.1.1
|
S2O32−
+ CN−
SO32− + SCN−
|
Cyanide poisoning
|
Ribonuclease
|
3.1.26.4
|
RNA hydrolysis
|
Antiviral
|
b-Lactamase
|
3.5.2.6
|
Penicillin
penicilloate
|
Penicillin allergy
|
Streptokinase c
|
3.4.22.10
|
Plasminogen
plasmin
|
Blood clots
|
Trypsin
|
3.4.21.4
|
Protein hydrolysis
|
Inflammation
|
Uricase d
|
1.7.3.3
|
Urate + O2
allantoin
|
Gout
|
Urokinase e
|
3.4.21.31
|
Plasminogen
plasmin
|
Blood clots
|
a Hyaluronoglucosaminidase
b thiosulphate
sulfurtransferase
c streptococcal cysteine proteinase
d urate oxidase
e
plasminogen activator
As enzymes are specific biological catalysts, they should
make the most desirable therapeutic agents for the treatment of metabolic
diseases. Unfortunately a number of factors severely reduces this potential
utility:
- They are too large to be distributed simply within the
body's cells. This is the major reason why enzymes have not yet been successful
applied to the large number of human genetic diseases. A number of methods are
being developed in order to overcome this by targeting enzymes; as examples,
enzymes with covalently attached external b-galactose residues are targeted at
hepatocytes and enzymes covalently coupled to target-specific monoclonal
antibodies are being used to avoid non-specific side-reactions.
- Being generally foreign proteins to the body, they are
antigenic and can elicit an immune response which may cause severe and
life-threatening allergic reactions, particularly .on continued use. It has
proved possible to circumvent this problem, in some cases, by disguising the
enzyme as an apparently non-proteinaceous molecule by covalent modification.
Asparaginase, modified by covalent attachment of polyethylene glycol, has been
shown to retain its anti-tumour effect while possessing no immunogenicity.
Clearly the presence of toxins, pyrogens and other harmful materials within a
therapeutic enzyme preparation is totally forbidden. Effectively, this
encourages the use of animal enzymes, in spite of their high cost, relative to
those of microbial origin.
- Their effective lifetime within the circulation may be
only a matter of minutes. This has proved easier than the immunological problem
to combat, by disguise using covalent modification. Other methods have also been
shown to be successful, particularly those involving entrapment of the enzyme
within artificial liposomes, synthetic microspheres and red blood cell ghosts.
However, although these methods are efficacious at extending the circulatory
lifetime of the enzymes, they often cause increased immunological response and
additionally may cause blood clots.
In contrast to the industrial use of enzymes,
therapeutically useful enzymes are required in relatively tiny amounts but at a
very high degree of purity and (generally) specificity. The favoured kinetic
properties of these enzymes are low Km and high Vmax in order to be maximally
efficient even at very low enzyme and substrate concentrations. Thus the sources
of such enzymes are chosen with care to avoid any possibility of unwanted
contamination by incompatible material and to enable ready purification.
Therapeutic enzyme preparations are generally offered for sale as lyophilised
pure preparations with only biocompatible buffering salts and mannitol diluent
added. The costs of such enzymes may be quite high but still comparable to those
of competing therapeutic agents or treatments. As an example, urokinase (a
serine protease, see Table 4.4) is prepared from human urine (some genetically
engineered preparations are being developed) and used to dissolve blood clots.
The cost of the enzyme is about £100 mg−1, with the cost of treatment in a case
of lung embolism being about £10000 for the enzyme alone. In spite of this, the
market for the enzyme is worth about £70M year−1.
A major potential therapeutic application of enzymes is in
the treatment of cancer. Asparaginase has proved to be particularly promising
for the treatment of acute lymphocytic leukaemia. Its action depends upon the
fact that tumour cells are deficient in aspartate-ammonia ligase activity, which
restricts their ability to synthesise the normally non-essential amino acid L-asparagine.
Therefore, they are forced to extract it from body fluids. The action of the
asparaginase does not affect the functioning of normal cells which are able to
synthesise enough for their own requirements, but reduce the free exogenous
concentration and so induces a state of fatal starvation in the susceptible
tumour cells. A 60% incidence of complete remission has been reported in a study
of almost 6000 cases of acute lymphocytic leukaemia. The enzyme is administered
intravenously. It is only effective in reducing asparagine levels within the
bloodstream, showing a half-life of about a day (in a dog). This half-life may
be increased 20-fold by use of polyethylene glycol-modified asparaginase.
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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