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Reversible polymerization of histidine ammonia lyase. The role of sulfhydryl groups in the activity and polymeric state of the enzyme.

Abstract Histidine ammonia lyase, obtained by a modification of the purification procedures previously described, is shown to be a mixture of enzymatically active polymeric species which undergo depolymerization on treatment with thiols. The monomeric unit of molecular weight 213,000 is itself composed of four subunits of molecular weight 55,000. The sulfhydryl groups on the enzyme play a role in catalysis as well as in polymerization. Spectrophotometric and circular dichroism measurements have revealed the presence of a hitherto undetected chromophore absorbing near 315 mµ. Inactivation of the enzyme with sodium borohydride or l-cysteine markedly changes the absorption of the protein in this spectral region.