Characteristics of alanine racemase in Lactobacillus sakeiZH-2 strain

Some d-amino acid functions for food production are widely known: d-alanine improves sensory evaluations of sake, beer, and fermented foods. Therefore, for the application of d-amino acids, alanine racemase (ALRase) in Lactobacillus sakei ZH-2, which has strong racemization, was analyzed using molecular biological methods. It had been hypothesized that ALRase coding DNA, alr, in ZH-2 strain differs from those of other Lactobacillus sakei strains. However, complete genome sequencing by the National Center for Biotechnology (NCBI) revealed the amino acid sequence of alr in ZH-2 strain to have homology of 99.4% similarity with the alr in Lactobacillus sakei 23K strain. However, it is considered that the sequence of alr was a unique amino acid sequence in the lactic acid bacteria group. DNA "alr" of ZH-2 strain has a 1140 bp DNA base with 41 kDa molecular mass. Its molecular mass was inferred as approximately 38.0 kDa using SDS-PAGE. Its optimum conditions are pH 9.0 at 30-40°C, showing stability at pH 9.0-10.0 and 4-40°C. Its cofactor is pyridoxal phosphate. Its activity is activated more by copper and zinc ions than by the lack of a metal ion. Additionally, its K m is 1.32 × 10-3 (mol), with V max of 4.27 × 10-5 (μmol-1 min-1). ALRase reacted against alanine most strongly in other substrates such as amino acids. The enzyme against serine was found to have 40% activity against alanine. The enzyme converted up to 54.5% of d-alanine from l-alanine ZH-2 strain.

The Reducing Capacity of Thioredoxin on Oxidized Thiols in Boiled Wort

Free thiol-containing proteins are suggested to work as antioxidants in beer, but the majority of thiols in wort are present in their oxidized form as disulfides and are therefore not active as antioxidants. Thioredoxin, a disulfide-reducing protein, is released into the wort from some yeast strains during fermentation. The capacity of the thioredoxin enzyme system (thioredoxin, thioredoxin reductase, NADPH) to reduce oxidized thiols in boiled wort under fermentation-like conditions was studied. Free thiols were quantitated in boiled wort samples by derivatization with ThioGlo1 and fluorescence detection of thiol-derivatives. When boiled wort was incubated with all components of the thioredoxin system at pH 7.0 and 25 °C for 60 min under anaerobic conditions, the free thiol concentration increased from 25 to 224 μM. At pH values similar to wort (pH 5.7) and beer (pH 4.5), the thioredoxin system was also capable of increasing the free thiol concentration, although with lower efficiency to 187 and 170 μM, respectively. The presence of sulfite, an important antioxidant in beer secreted by the yeast during fermentation, was found to inactivate thioredoxin by sulfitolysis. Reduction of oxidized thiols by the thioredoxin system was therefore only found to be efficient in the absence of sulfite.