الفهرس 
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Abstract
Peroxidase enzymes are used in extensive potential clinical, biochemical, biotechnological and industrial applications. An ideal peroxidase for large-scale biocatalysis possesses wide substrate specificity, and has better stability
over a wide range of pH, temperature and sensitivity.
Therefore, the search for such novel peroxidases is of immense research interest. This study has focused on the purification and characterization of a new cationic peroxidase from Euphorbia tirucalli latex, which possessed high levels of
peroxidase activity, to be a good inexpensive starting material for peroxidase production. The findings obtained can be summarized in the following:
1- E. tirucalli latex peroxidase (ELP) activity was
quantitatively determined in the aqueous layer of E. tirucalli latex benzene extract to be 1303 units/mg protein,which indicated that E. tirucalli latex could be potentially a good source for peroxidase.
2- ELP was purified to homogeneity as judged by SDSpolyacrylamide gel electrophoresis. The purification scheme involved benzene extraction, gel filtration using Sephacryl S-200 column and ion-exchange on CMSepharose column. The purified ELP had a specific activity of 2621 units/mg protein, with 2 fold purification over the
crude extract (aqueous layer of benzene extract).
3- The molecular weight of ELP was found to be ~ 44 kDa as calculated from the calibration curve of Sephacryl S-200 column. This value was confirmed by SDS-PAGE (44KDa), where the enzyme migrated as single protein band
suggesting that ELP is a monomeric enzyme.
4- The inhibitory effect of different compounds for ELP indicated that the most potent inhibitors for ELP were sodium azide, DTT and EDTA where 1 mM caused about complete inhibition for them. from the above results, it can be concluded that ELP had Fe3+ and Ca2+ atoms in the
active site and it could be a heme peroxidase.
5- The effect of pH on the ELP activity revealed that it had an optimum pH at 7.
6- The effect of temperature on ELP indicated that it had anoptimum temperature at 40 °C. Heat stability studies on ELP revealed that the enzyme was stable up to 50 °C, then the stability decreased by increasing the temperature where the enzyme lost 13% of its activity at 60 °C followed by a
loss of 70% at 70 °C, and a complete inhibition at 80°C.
7- The effect of different metal cations on the activity of ELP revealed that all the examined cations showed different partial inhibitory effects on the activity of ELP, except for Ca2+ which resulted in enzyme activation of the enzyme by
119% and 370% at 1 and 5 mM, respectively, as well as,Mg2+ that caused a slight activation of 12% at 1 mM concentration. The most potent inhibitors for ELP were Hg2+ and Cu2+ at the least concentration used (1 mM).
8- The substrate specificity of ELP was determined for a number of substrates. The affinity decreased in the order of ABTS > guaiacol > OPD > AAP.
9- Km values of ELP were determined for ABTS, guaiacol,H2O2, AAP and OPD and found to be 0.503, 4.4, 15, 25 and 33.4 mM, respectively.
10- The thermal inactivation of ELP in the absence and presence of Ca2+ ions, effect of Ca2+ concentration on ELP activity, and effect of CaCl2 addition on ELP activity before and after incubation at 70 °C were studied. The results
indicated that Ca2+ ions had a protective role in thermal inactivation of ELP and act as a stabilizer and an activator.