| Abstract
| - The coding region of the copper/zinc superoxide dismutase (Cu/Zn SOD) cDNA from papaya fruit,Carica papaya L. cv. Tainong 2, was cloned into an expression vector, pET-20b(+). The Cu/Zn SODwas expressed in Escherichia coli and purified by His-tag technique. Two active forms of the enzyme(30% dimer and 70% monomer) in equilibrium were observed. The activity of the dimeric enzymewas higher than that of the monomeric form. The thermal inactivation rate constant Kd valuescalculated for the dimer and monomer at 90 °C were −0.0203 and −0.0216 min-1, and the half-lives for inactivation were 41.9 and 31.8 min, respectively. This indicated that the dimeric enzymewas more stable than its monomeric form. The dimerization of the enzyme was inhibited underacidic pH (below 3.0) or imidazole buffer (above 0.5 M), whereas it was not affected under alkalinepH (above 9.0). Both activity and forms of the enzyme were not affected by 1−4% SDS. Furthermore,the dimeric enzyme was much more resistant to proteolytic attack after 3 h of incubation at 37 °Cwith trypsin or chymotrpsin. In addition, mutation of the papaya Cu/Zn SOD at position 48 fromLeu to Phe (L48F) affected the association of monomer, whereas a mutant with Lys substitution(L48K) at the same position tended to dissociate into monomeric form. Keywords: Cu/Zn SOD; superoxide dismutase; papaya fruit; Carica papaya L. cv. Tainong 2;dimer−monomer equilibrium shift
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