Sulfhydryl groups of chicken liver purine nucleoside phosphorylase.

Purine nucleoside phosphorylase purified from chicken liver was inactivated by the sulfhydryl reagent, 4,4'-dithiodipyridine. The inactivation was strongly prevented by the substrates, inosine, phosphate and arsenate. Inactivated enzyme could be reactivated by dithiothreitol. The stoichiometry of modification of sulfhydryl groups was 1.2 residues per mol of the enzyme subunit. The enzyme had five sulfhydryl groups per subunit. One of the five sulfhydryl groups was very reactive with 4,4'-dithiodipyridine, three groups were reactive with excess, 4,4'-dithiodipyridine and another one was masked.