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  • Title: Histidine decarboxylase of Lactobacillus 30a: function and reactivity of sulfhydryl groups.
    Author: Lane RS, Snell EE.
    Journal: Biochemistry; 1976 Sep 21; 15(19):4175-9. PubMed ID: 9132.
    Abstract:
    Two classes of sulfhydryl groups in histidine decarboxylase from Lactobacillus 30 a can be differentiated by their reaction with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). Five cysteinyl residues (class I) of the native enzyme are titrated by DTNB as the pH of the reaction medium is increased from 6.5 to 7.5; the pH-rate profile for their reaction is described by a pKa of 9.2. An additional five thiol groups (class II) are titrated only when denaturing agents are added above neutral pH. Histidine decarboxylase is completely inactivated by DTNB in a kinetically second-order process (Kapp = 660 +/- 20 M-1 min-1 at pH 7.6 and 25 degrees C) which occurs coincident with and at the same rate as modification of the five class-I SH groups of the enzyme, i.e., one thiol group per pyruvoyl prosthetic group. The competitive inhibitors, histamine and imidazole, markedly enhanced the reactivity of these cysteinyl residues toward DTNB; this enhancement is accompanied by a concomitant increase in the rate of inactivation. A single SH group in each of the five catalytic units of histidine decarboxylase is thus implicated as being critical for the expression of enzymatic activity.
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