Primary structure from amino acid and cDNA sequences of two Cu,Zn superoxide dismutase variants from Xenopus laevis

A mixture of two different amino acid sequences was discovered in Cu,Zn superoxide dismutase purified from the amphibian Xenopus laevis. No N-terminal post-translational modification was found. The high number of substitutions in the sequence suggested that protein heterogeneity was a product of gene duplication. This was confirmed by isolation of two different cDNA clones. Nucleotide sequence analysis allowed the primary structure of the two peptide chains to be unambiguously assigned. The observed changes (19 in 150 residues) are distributed along the peptide chain to give similar protein net charges although substitutions of the same polarity and/or charge were the exception rather than the rule. The degree of diversity between the two Xenopus variants is comparable to that between mammalian sequences and shows that the putative increase of the rate of mutation for Cu,Zn superoxide dismutase at later evolution stages (Y. M. Lee et al., 1985, Arch. Biochem. Biophys. 241, 577-589; G. J. Steffens et al., 1986, Biol. Chem. Hoppe-Seyler 367, 1017-1024) is observed in amphibians. This is the first time complete sequences for Cu,Zn superoxide dismutase variants from the same organism have been found to be products of divergent genes and not simply allelic mutations.