CHARACTERIZATION OF SULFHYDRYL GROUPS OF ACTIN

Actin contains 5 cysteine residues for 44,000 daltons. The number of sulfhydryls was determined with silver nitrate and back-titration of excess silver with glutathione. On treatment of native and urea-denatured actin with a new reagent, 2,2′-dicarboxy-4′-iodoacetamidoazobenzene, three and four sulfhydryl groups, respectively, were found to react. The corresponding sulfhydryl peptides were located by the fingerprint method. They were isolated and their amino acid compositions were determined. Since actin with the azo dye coupled to its three surface sulfhydryl groups can still polymerize it seems that these sulfhydryl groups are probably not directly involved in the polymerization reaction. However, two of these sulfhydryl groups are covered in the course of polymerization. G-actin binds three molecules of salyrgan (O-carboxymethylsalicyl[3-oxymercuri-2-methoxypropyl]amide-203Hg). Th is organomercurial inhibits polymerization. Evidence is presented that salyrgan binds to the same three surface SH groups to which the azo dye binds. In F-actin only one of these three sulfhydryls reacts with the iodoacetamide derivative. This single sulfhydryl group becomes inaccessible in the course of actomyosin formation, but plays no role in the binding of actin to myosin. F-actin containing 0.87 mole of azo dye/mole of actin is still reactive with myosin, even with the azo dye bound to this cysteine residue. Furthermore the complex formed with the derived F-actin undergoes the same viscosity changes as native actomyosin. © 1969, American Chemical Society. All rights reserved.