Sequence homology between potato and rabbit muscle phosphroylases. Isolation of cysteinyl peptides by covalent chromatography from the potato enzyme and their amino acid sequences.

To elucidate the structural similarity between alpha-glucan phosphorylases from different sources, the amino acid sequences of the cysteinyl regions in potato phosphorylase were determined, and compared with the complete sequence of the rabbit muscle enzyme. Cysteinyl peptides were purified by covalent chromatography based on thiol-disulfide exchange. Potato phosphorylase was coupled to Thiopropyl-Sepharose 6B in the presence of urea, and, after tryptic digestion, 10 distinct cysteinyl peptides which accounted for all the cysteine present in the enzyme were finally isolated in high yields. From the sequence comparison, all of the peptides surrounding the cysteinyl residues in the potato enzyme are homologous with widely distributed specific regions in the rabbit muscle enzyme, although only 4 of 10 cysteinyl residues are conserved between the two enzymes. These observations, in addition to the previously established homology in the cofactor site, show that potato and rabbit muscle phosphorylases are similar in terms of the primary structure. The different properties of cysteinyl residues in the two enzymes are discussed based on the present sequence comparison and the recent X-ray crystallographic data for the rabbit muscle enzyme.