Inhibition of thiaminase I from Bacillus thiaminolyticus. Evidence supporting a covalent 1,6-dihydropyrimidinyl-enzyme intermediate

Thiaminase I from Bacillus thiaminolyticus strain Matsukawa et Misawa is completely and irreversibly inhibited by treatment with 4-amino-6-chloro-2-methylpyrimidine. Inhibition is a time-dependent first-order process, exhibiting a half-time of 4 h at an inhibitor concentration of 5 mM. A specific active-site-directed inactivation is supported by protection of the enzymatic activity in the presence of the substrates thiamin and quinoline as well as by the observation that a stoichiometric amount of inorganic chloride is released during inactivation. 4-Amino-5-(anilinomethyl)-6-chloro-2-methylpyrimidine, which resembles the structure of the product of base exchange of thiamin with aniline, inactivates thiaminase approximately 2 orders of magnitude faster. Inactivation is again complete and irreversible and is a time-dependent first-order process, in this case exhibiting saturation at low inhibitor concentrations (KI = 96 microM). Enzyme inactivation can be explained as the result of displacement of chloride from the chloropyrimidine by a nucleophile at the enzyme active site. The inactivation suggests that the Zoltewicz-Kauffman model of bisulfite-catalyzed thiamin cleavage [Zoltewicz, J. A., & Kauffman, G. M. (1977) J. Am. Chem. Soc. 99, 3134-3142], which calls for the reversible nucleophilic addition of catalyst across the 1,6 double bond of thiamin's pyrimidine ring, may be applicable to thiaminase as well.     

Cell-Bound Thiaminase I of Bacillus thiaminolyticus

The distribution of the extracellular enzyme, thiaminase I, was determined for logarithmically growing cultures of Bacillus thiaminolyticus. About 60% of the enzyme is associated with the cells throughout the growth cycle. The remainder of the enzyme is in the culture medium. The release of the cell-bound thiaminase I is examined under a variety of conditions. The rate and extent of release is dependent on the pH and the nature of the incubation solution. The release process appears to be relatively independent of de novo protein synthesis, energy derived from oxidative phosphorylation, or divalent metal ions. The absence of carbon or nitrogen sources has little effect on the release of the enzyme. Cell-bound thiaminase I probably is the immediate precursor for extracellular thiaminase I found in the culture medium. Washed cells continue to release thiaminase I at the expense of cell-bound enzyme. In addition, purified cell-bound thiaminase I is indistinguishable from purified extracellular thiaminase I by a number of physical and kinetic criteria.     

Reversible Inactivation of Thiaminase I of Bacillus thiaminolyticus by Its Primary Substrate, Thiamine

Thiaminase I of Bacillus thiaminolyticus is reversibly inactivated when it is incubated with its primary substrate, thiamine, or with one of several structural analogues of thiamine in the absence of an acceptor base. The inactivation reaction is pH and temperature dependent and is stochiometric with respect to thiamine and thiaminase I concentrations. One molecule of thiamine is cleaved for each molecule of enzyme inactivated. Inactivation is prevented or reversed by sulfhydryl-reducing agents. Active or reactivated thiaminase I migrate as a single band in polyacrylamide electrophoresis gels. Inactive thiaminase I appears to migrate as two separate bands. Active, inactive, and reactivated thiaminase I are immunologically similar. A possible mechanism for the inactivation of thiaminase I by its substrate is discussed.     

Molecular studies on thiaminase I

The thiaminase I gene of Bacillus thiaminolyticus was cloned on a 1.6 kb DNA fragment (enzyme molecular weight 42 000), and was expressed in both Escherichia coli and Bacillus subtilis. When a selection drug was absent, the plasmid was maintained stably for approx. 100 generations in wild-type E. coli. Instability of the thiaminase gene was demonstrated in the thiamin pyrophosphate-requiring mutant of E. coli from which the plasmid was deleted rapidly. Wild-type E. coli accumulated the enzyme in its periplasm. A method for the detection of thiaminase I enzyme in SDS-polyacrylamide gel was developed. Thiaminase I of B. thiaminolyticus was found to exist in two sizes, 44 and 42 kDa, among different strains. Moreover, thiaminase of 42 kDa became approximately 41 kDa after a long-term culture, most likely because of the action of proteinases. Thiaminase expressed in E. coli from a thiaminase-positive recombinant plasmid was 42 kDa, and showed the same mobility on SDS-polyacrylamide gele electrophoresis as the enzyme isolated from the young culture of the parent strain of B. thiaminolyticus used for cloning. This value was, therefore, considered to represent intact thiaminase that had escaped from the attack of bacilli proteinases.     

Solubilization of coat protein from Bacillus thiaminolyticus spores.

Solubilization of spore coat protein of Bacillus thiaminolyticus was investigated using various reagents, and partial characterization of solubilized protein was carried out. Five per cent of the sodium dodecyl sulphate (SDS) treatment was the most effective for solubilization of coat protein, and 5% SDS + 8 M urea and 0.06 N NaOH were also useful. Acrylamide gel disc electrophoresis indicated that the SDS-soluble fraction mainly consists of a single band of protein and its molecular weight was estimated at about 15,000. The SDS+ urea-soluble fraction comprised two proteins with a molecular weight of 14,500 and 32,000, and an alkali-soluble fraction of 12,000 and 25,000 respectively.     

Bacillus thiaminolyticus sp. nov., nom. rev

The name "Bacillus thiaminolyticus" Kuno 1951 was not included on the Approved Lists of Bacterial Names and has lost standing in bacteriological nomenclature. The genetic homogeneity of "Bacillus thiaminolyticus" was assessed by determining guanine-plus-cytosine contents by the buoyant density method and by measuring DNA relatedness by using spectrophotometric reassociation procedures. Of the 26 strains which I studied, 24 had guanine-plus-cytosine contents in the range from 52 to 54 mol%. The consistently high DNA relatedness values of 60 to 100% of these 24 strains to the type strain indicated that the "B. thiaminolyticus" group is genetically homogeneous. Low DNA relatedness values of 20 to 31% showed that "B. thiaminolyticus" is genetically unrelated to Bacillus alvei, "Bacillus aneurinolyticus," "Bacillus apiarius," Bacillus larvae, Bacillus laterosporus, Bacillus macerans, and Bacillus stearothermophilus. In general, the "B. thiaminolyticus" group was highly homogeneous for 49 phenotypic characteristics and clearly distinguishable from B. alvei, with which it was allegedly synonymous. On the basis of these findings, revival of the name Bacillus thiaminolyticus is proposed.     

Production of thiaminase I for analytical purposes

A simple medium enhancing the production of thiaminase I (EC 2.5.1.2) by Bacillus thiaminolyticus was developed. Ca²⁺ stimulated the enzyme production. The activity of extracellular thiaminase I ranged between 1.29 and 1.33 U/ml medium.     

Changes of Ultrastructure in Spore Coat of Bacillus thiaminolyticus during Germination and Outgrowth

Electron microscopic observation showed that the spore coat of Bacillus thiaminolyticus consisted of at least four layers; a high electron dense outer spore coat layer with five prominent ridges, a middle spore coat layer including two layers of a high and a low electron density, and an inner spore coat layer composing six to seven laminated layers. Rapid breakdown of the cortex and swelling of the core occurred in spores which were allowed to germinate by L -alanine for 45 min, whereas no change of surface feature was observed by scanning electron microscopy. Germination and outgrowth of spores in nutrient broth proceeded, being accompanied by morphological changes, in three steps; the first is a rapid breakdown of the cortex and swelling of the core, the second degradation of the inner layer at a prominent region of the spore coat, and the last rupture of the spore coat and emergence of a young vegetative cell.     

Effect of alanine-containing dipeptides on germination of Bacillus thiaminolyticus spores.

Stereoisomeric alanylalanine (Ala-Ala) derivatives were examined for their effects on germination of Bacillus thiaminolyticus spores. L-Ala-L-Ala and L-Ala-glycine were effective in inducing germination, and their activities were completely inhibited by D-Ala. L-Ala-D-Ala and glycine-D-Ala competitively prevented L-Ala-induced germination. Sarcosine- or beta-Ala-containing L-alanyldipeptides and eight kinds of alanyltripeptides did not show any detectable effect on germinability or any inhibitory effect. No detectable amounts of Ala were found in germination exudates when alanylpeptides were incubated with spores. The ability of these peptides to induce or inhibit germination depends on their steric conformation and a certain distance between the primary amino group and the free carboxyl groups. Involvement of L-Ala dehydrogenase in the initiation of germination is unlikely because L-Ala-L-Ala was not a substrate and L-Ala-D-Ala was not an effective inhibitor of enzyme activity.     

An assay for thiaminase I in complex biological samples

An alternative method for measuring thiaminase I activity in complex samples is described. This assay is based on the selective consumption of the highly chromophoric 4-nitrothiophenolate by thiaminase I, resulting in a large decrease in absorbance at 411nm. This new assay is simple and sensitive, and it requires only readily available chemicals and a visible region spectrophotometer. In addition, the assay is optimized for high-throughput analysis in a 96-well format with complex biological samples.     

Phenol degradation by Paenibacillus thiaminolyticus and Bacillus cereus in axenic and mixed conditions

In this study, seven aerobic bacterial strains were screened for phenol tolerance at different concentration of phenol. Bacterial strains were unable to utilize phenol in absence of glucose, indicated the phenomenon of co-metabolism. Among the seven isolated bacterial strains, only ITRC BK-4 and ITRC BK-7 found potential and identified as Paenibacillus thiaminolyticus (DQ435022) and Bacillus cereus (DQ435023), respectively. Phenol degradation was monitored routinely with spectrophotometer and further confirmed by HPLC analysis. ITRC BK-4, ITRC BK-7 and mixed culture degrade 700 ppm phenol up to 51.72, 70.00 and 84.57% respectively in mineral salt medium (MSM) at temperature 37 ± 1°C, pH 7.5 ± 0.2, 120 rpm in presence of 1% glucose (w/v) within 144 h incubation. The mix culture was found more potential for phenol degradation compared to axenic strains. Hence, the axenic and mixed strains of these bacteria would be useful for the removal/mineralization of phenol from industrial waste waters.     

Changes of ultrastructure in spore coat of Bacillus thiaminolyticus during germination and outgrowth.

Electron microscopic observation showed that the spore coat of Bacillus thiaminolyticus consisted of at least four layers; a high electron dense outer spore coat layer with five prominent ridges, a middle spore coat layer including two layers of a high and a low electron density, and an inner spore coat layer composing six to seven laminated layers. Rapid breakdown of the cortex and swelling of the core occurred in spores which were allowed to germinate by L-alanine for 45 min, whereas no change of surface feature was observed by scanning electron microscopy. Germination and outgrowth of spores in nutrient broth proceeded, being accompanied by morphological changes, in three steps; the first is a rapid breakdown of the cortex and swelling of the core, the second degradation of the inner layer at prominent region of the spore coat, and the last rupture of the spore coat and emergence of a young vegetative cell.