The secondary and tertiary structure of the oligomeric arginase (EC 3.5.3.1) from beef liver was investigated by circular dichroism (CD) and fluorescence measurements. The far‐ultraviolet CD spectrum of the enzyme at neutral pH is indicative of high helical content. The intrinsic fluorescence emission of the protein is due to tryptophan, the contribution of tyrsoine being small. Upon excitation at 295 nm, the maximum of emission occurs at 330 nm, implying that the trytophan residues are rather buried in a hydrophobic interior of the protein. Ethylenediaminetetraacetic acid (EDTA), which inactivates the enzyme by removing the functional Mn2+‐ion from the enzyme, does not dissociate the enzyme into subunits, nor affect noticeably its secondary and tertiary structure. Inactivation occurs in the acid pH range, being complete at pH below 4. However, acidification up to pH 1.5 produced only limited changes in the far‐ultraviolet CD spectrum and intrinsic fluorescence emission properties. The enzyme shows noteworthy thermal stability, as shown by measuring the residual activity after heating and by evaluating the temperature dependence of the CD signal at 220 nm and the intensity of emission fluorescence. A temperature of half inactivation (Tm) of 77° was determined upon heating the enzyme at pH 7.5 in the presence of Mn2+‐ions for 10 min; in the presence of EDTA, Tm is shifted to 55°. Taken together, these observations indicate that the structural stability of beef liver arginase arises from a clustering of hydrophobic amino acids and from Mn2+‐ion binding.
The enzyme arginase, purified from bovine liver, was covalently immobilized by the glutaraldehyde method to the inner surface of Cuprophan hollow fibers of a conventional hemodialyzer with a surface 1.3 m3. The yield of the process was 0.3 microgram/cm2 of active enzyme at physiological pH. The immobilization method did not adversely affect the physical and mechanical properties of hollow fibers neither their hemocompatibility. After sterilization with ethylene oxide, the bioreactor was able to metabolize five liters of 50 microM arginine solution at pH 7.4, in six hours.
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