Expression of the ansZ gene encoding a putative L-asparaginase II (BsAII) from Bacillus subtilis in Escherichia coli was examined. No expression was detected in E. coli transformed with a plasmid containing the full-length ansZ gene. Three N-terminal truncated enzymes (BsAIIT18M, BsAIIS40M, and BsAIID49M) were prepared based on comparison with the N-terminal sequences of other type II L-asparaginases. BsAIIT18M became easily inactivated during DEAE-Toyopearl column chromatography. The purified N-terminal-truncated enzymes BsAIIS40M and BsAIID49M had tetrameric subunit structures and V max values of 45.5 and 45.8 U/mg towards L-asparagine, respectively. Their K m values were 2.06 and 7.02 mM, respectively. The enzymes differed from asparaginase II from E. coli and Erwinia carotovora in substrate specificity and affinity for L-asparagine. BsAIIS40M and BsAIID49M retained over 80% of their original activities in the presence of 15% NaCl, and thus may find application in the food industry for products in which NaCl is used. This study also revealed that BsAII is rather different from the type I enzyme (BsAI) from B. subtilis in substrate specificity and salt-tolerance.
It has been reported that acrylamide, a potential carcinogen, is formed from the reaction of L-asparagine (L-Asn) and reducing sugars contained in foods during heating processes and free asparagine is a limiting factor for acrylamide formation. It has been reported that potato products such as potato chips, which are made through heating processes, contain high levels of acrylamide. To decrease the amount of L-Asn in potatoes using L-asparaginase, effective treatment conditions of sliced potatoes with the enzyme have been investigated. By treating sliced potatoes with Bacillus subtilis L-asparaginase II (BAsnase; 4 U/g potato), appriximately 40 % of L-Asn in the sliced potatoes was converted into L-aspartic acid (L-Asp). To make this enzyme more effective, prior to enzymatic treatment, sliced potatoes were freeze-thawed, dried at 90 °C for 20 min, and vacuum treated for 10 min under decompressed condition, resulting in the hydrolysis of approximately 90 % of L-Asn to L-Asp. The acrylamide content of BAsnase-treated fried potato chips decreased to below 20 % of that of BAsnase-untreated fried potato chips. Treatment conditions examined in this study were found to be effective to suppress the formation of acrylamide in fried potato chips.
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