Amino Acid Sequence Studies of Horseradish Peroxidase. I. Tryptic Peptides

Commercially available horseradish peroxidase (RZ 3.1) was characterized with respect to its homogeneity by (1) chromatography on CM-cellulose, (2) disc gel electrophoresis in alkaline and acidic buffers, (3) micro-scale sucrose gradient isoelectrofocusing in pH 3–10 and pH 8–10 gradients, (4) gel isoelectrofocusing in pH 3–10 and pH 8–10 gradients, and (5) amino acid and hexosamine analyses. The preparation was found to be highly homogeneous except by pH 8–10 gel isoelectrofocusing which resolved it into several very close bands. This heterogeneity has been assumed to reflect differences in carbohydrate composition rather than in the amino acid sequence or composition. Amino acid analyses after performic acid oxidation yielded eight cysteic acid residues per mole of enzyme. Since no S-carboxymethylcysteine was recovered after treatment of the protein in 8 M urea with iodoacetic acid, it was concluded that the enzyme has four disulfide bridges. Peptides resulting from a tryptic digest of the heme-free enzyme were purified by high-voltage paper electrophoresis and subjected to sequence analysis. Several half-cystine sequences were elucidated after isolation of the radioactive peptides from a tryptic digest of the reduced and 14C-S-carboxymethylated protein. The complete sequences of 21 and partial sequences of three tryptic peptides were determined. These account for 203 of the approximately 300 amino acid residues of this protein. Several sites of carbohydrate attachment were observed.