Some enzymes are employed as human digestive aids to replace or increase endogenous digestive enzymatic activities. Lactase and pancreatic enzyme replacement therapies are two such examples. The enzyme P-D-galactoside-galactohydrolase (EC, commonly known as "P-galactosidase" or "lactase," hydrolyzes lactose, forming glucose and galactose [165]. Lactose, or milk sugar, is a disaccharide found in the milk of most mammals. Lactose is digested in vivo in the gastrointestinal tract by lactase-phlorizin hydrolase, a membrane-bound enzyme of the small intestinal epithelial cells. Intestinal lactase insufficiency results in lactose intolerance (maldigestion with negative clinical symptoms). Intestinal P-galactosidase insufficiency can be a result of downregulation in the expression of the P-galactosidase gene, which is a penetrant autosomal gene located on chromosome

2q21, or by injury to the intestinal mucosa [166]. The incidence of P-galactosidase deficiency has been reported to be as large as approximately 75% of the world's population, and is most prevalent among the populations of Asia and the United States [167].

P-Galactosidase-deficient populations have difficulty in consuming milk and other lactose-containing products, as ingestion of lactose can result in abdominal pain, diarrhea, and flatulence. Intestinal P-galactosidase insufficiency is also thought to be a possible etiology for infantile colic. P-Galactosidase has been widely studied, with incidence of its occurrence reported in animal organs, plants, and microorganisms [168]. Microbial P-galactosidases are most extensively utilized for commercial purposes, owing to their high levels of production and desirable physicochemical properties (e.g., pH and temperature optima) [169]. The principal enzymes exploited commercially are obtained from GRAS-listed yeasts and fungi such as Kluyveromyces lactis and Aspergillus oryzae [170]. The P-galactosidase enzyme derived from Aspergillus oryzae is an extracellular protein. The enzyme has a molecular mass of 105 kDa, is a homodimer, and is glycosylated. The enzyme has a pH optimum of 4.5 and also displays transglycosylation activity. It does not require cofactors or metal ions for activity unlike yeast P-galactosidase [171].

Lactose intolerance and infantile colic are two lactose-related conditions whose symptoms can be relieved by dietary hydrolysis of the lactose load via the use of exogenous P-galactosidase enzyme [172-177]. Lactase digestive aids are commonly available as commercial products in pharmacies and health food stores. Some of the most popular products include Lactaid, Dairycare, Lacteeze, and Lifeplan (Table 13.2). The majority of these digestive aids contain Aspergillus-derived P-galactosidases. Studies have been carried out in vivo to determine the efficacy of lactase digestive aids derived from various sources in alleviating lactose maldigestion and intolerance. Varying results have been obtained with different enzyme administration strategies, but a positive effect was evident for all [177]. In addition, clinical studies assessing the efficacy of lactase-based treatments for colic have reported favorable results in alleviating the condition [172,176].

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