Crystalline l‐Histidine Ammonia‐Lyase of Achromobacter liquidum

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Abstract
Crystalline l‐histidine ammonia‐lyase of Achromobacter liquidum was prepared with a 24% recovery of the activity. The specific activity of the pure enzyme (63 μmol of urocanic acid min−1 mg−1) is similar to those so far reported for the enzyme from other sources. The purified enzyme appeared to be homogeneous by analytical disc electrophoresis and isoelectric focusing (pI= 4.95). The molecular weight determined by Sephadex G‐200 gel filtration is 200 000. The optimum pH is 8.2, and the optimum temperature is 50 °C. The enzyme showed strict specificity to l‐histidine (Km= 3.6 mM). Several histidine derivatives are not susceptible to the enzyme but do inhibit the enzyme activity competitively; the most effective inhibitors are l‐histidine methyl ester (Ki= 3.66 mM) and β‐imidazole lactic acid (Ki= 3.84 mM). l‐Histidine hydrazide (Ki= 36 mM) and imidazole (Ki= 6 mM) noncompetitively inhibited the enzyme. EDTA markedly inhibited enzyme activity and this inhibition were reversed by divalent metal ions such as Mn2+, Co2+, Zn2+, Ni2+, Mg2+, and Ca2+. These results suggest that the presence of divalent metal ions is necessary for the catalytic activity of histidine ammonia‐lyase. Sodium borohydride and hydrogen peroxide inhibited the enzyme activity.

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