Lactose (Lac), a by product in the dairy industry that is available cheaply and abundantly, has been utilized for baby food, confections, and galactose glucose syrup as a sweetener. However, surplus lactose corresponding to more than 2.9 million tons a year has been merely wasted.LA has several attractive properties. High water solubility allows the practical utilization of LA as a water soluble antibiotic, erythromycin lactobionate. LA is also utilized as a firming agent for foods, a moisturizing agent for cosmetics, and a component in preservative solution for the transplantation of organs. 8) LA is also expected to have usefulness as a functional oligosaccharide to promote the intestinal absorption of minerals 9) and the growth of bifidobacteria 10) as well. Such practical and potential usefulness led us to investigate the biological oxidation of Lac for an alternative production process of LA.An oxidized product of Lac, lactobionic acid (LA, O D galactosyl D gluconic acid), has been reported to have several useful physical and physiological properties; however LA has been supplied for limited usage, because there is no practical way to produce it other than chemical oxidation. Biological oxidation of Lac has not received as much attention because no appropriate biocatalysts have been developed: Conventional Glc oxidase (EC 1.1.3.4) 1 3) acts specifically on D Glc but not on Lac at all. Oxidases from red alga, 4) citrus fruits, 5) and Acremonium strictum 6) have been reported to oxidize Lac to form lactobiono lactone which spontaneously hydrolyzed to LA in aqueous solution, while the activity and productivity were unsatisfactory, and moreover, the enzymes were less active toward Lac compared to other saccharides. Glc dehydrogenase (EC 1.1.1.47) from Pseudomonas graveolence 7) oxidizes Lac using FAD as a proton acceptor. This coenzyme nevertheless should be reproduced to obtain LA in significant yields. We directed our attention to microorganisms and enzymes, which are able to oxidize Lac for the utilization of surplus lactose. We reported a bacterial isolate belonging to Burkholderia cepacia possessing not only considerable intracellular oxidase activity on Lac, but also lacking galactosidase activity to decompose Lac and LA, both of which facilitated considerable yields of LA in the fermentation process.11) Recently, the accumulation of approximately 40% (w v) LA in broth within 240 h was attained by the fed batch culture of a mutant (No. 24) with improved tolerance to high Lac concentrations.
12)In this study, Lac was converted by the resting cells of B. cepacia No. 24. Advantages of the presented process involved easy separation of the product, a calcium salt of LA, and a repeated use of cells as a valuable catalyst. Takehara
A lactose-oxidizing enzyme was obtained from culture supernatant of a fungal strain of Paraconiothyrium sp. KD-3. The enzyme was a flavoprotein with a molecular mass of 54 kDa. The purified enzyme oxidized various monosaccharides and oligosaccharides such as D-glucose, D-galactose, D-xylose, cellooligosaccharides and maltooligosaccharides in addition to lactose, using molecular oxygen as a good electron acceptor, to accumulate the corresponding aldonic acids and hydrogen peroxide. The Paraconiothyrium enzyme was suitable for the production of calcium lactobionate compared with a commercial hexose oxidase owing to the stability during the conversion. The enzyme converted 10 20% (w v) lactose completely to calcium lactobionate in a 500-mL reactor under aeration, agitation and pH regulation at 5.5. The neutralization was successfully performed by the addition of 10% (w w) slurry of calcium carbonate, while neutralization with 25% (w v) sodium hydroxide solution inactivated the enzyme gradually. The supplement of catalase to the mixture promoted the conversion by degrading hydrogen peroxide. The immobilized enzyme, which was prepared by the adsorption to a cation exchange resin followed by the condensation with carbodiimide, oxidized 18.5% (w v) lactose to produce calcium lactobionate in a batch reaction. Calcium lactobionate and aldonates derived from D galactose, D-xylose and L-arabinose showed high aqueous solubility and low calcium binding capability.
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