Structural studies of two apyrases from Solanum tuberosum

The proportion of acid and basic amino acid residues obtained for two homogeneous isoenzymes of apyrase isolated from different clonal varieties of Solanum tuberosum (Pimpernel and Desirée) was essentially the same. This does not agree with the difference in pI values observed. Treatment with asparaginase and glutaminase caused partial inactivation of both enzyme activities in both isoenzymes, and pI values were changed, but not equalized. The differences in pI values of the native isoenzymes may still be attributed to different proportions of glutamine and asparagine in the primary structure. Leucine is the amino-terminal residue in both isoenzymes. Both have two disulphide bridges and one buried sulphydryl group which is not essential for enzyme activity. Differences in pI values should thus be attributed to factors other than amino acid composition.