pH studies are used to gain insight into chemical mechanisms of enzyme catalysed
reactions. However, perhaps the most important practical point that is often overlooked in
pH studies is control of the ionic strength of reaction mixtures at the various pH values.
For example, cathepsins Band L were suspected to be involved in cancer invasion but pH
vs activity profiles indicated that they were not active at the extracellular pH (pH 7.2).
When these profiles were re-evaluated in buffers of constant ionic strength, as opposed to
buffers of constant molarity, it was shown that the enzymes were indeed active at pH 7.2.
Other enzymes have also been reported to be sensitive to ionic strength. These include
neutrophil elastase, class sigma glutathione S-transferase and penicillin G-acylase amongst
others.
The effects of increasing ionic strength on the activity of six enzymes were investigated.
a-Glucosidase (from bakers ' yeast), elastase (human leukocyte) and trypsin (bovine
pancreatic), cathepsin L (sheep liver), cathepsin B (rabbit liver), fruit bromelain (pineapple
fruit) were subjected to different ionic strength buffers and their activities and Km and Vmax
were determined as a function of ionic strength. The influence of ionic strength on Ki
values has not been previously reported and was also studied, using the interaction
between chicken egg-white cystatin C and cathepsin L as a model.
a-Glucosidase was found to have an ionic strength optimum and elastase showed
increasing activity with an increase in ionic strength. Trypsin activity decreased with
increasing ionic strength with a substrate containing a positively charged side chain in the
P1 position, and an increase in activity with a substrate containing a hydrophobic group at
the P1 position. Cathepsin B activity increased when acting on the substrate Z-Phe-ArgNHMec
and decreased when acting on Z-Arg-Arg-NHMec, with increasing ionic strength.
Bromelain showed an increase in activity with increasing ionic strength. Cathepsin L
activity decreased at increasing ionic strength and the Ki values for the cathepsin L-cystatin
C interaction increased with increasing ionic strength. The results obtained can be
attributed to the nature of the specificity pockets involved in binding the substrate, effects
on the catalytic mechanism of the enzyme or structural changes due to increasing ionic
strength. These results show that the ionic strength is a significant variable and should be
kept constant or at in vivo levels when assaying enzymes. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2005.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5558 |
| Date | January 2005 |
| Creators | Chuntharpursat, Eulashini. |
| Contributors | Dennison, Clive. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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