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β-galactosidase, also called beta-gal or β-gal, is a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides. Substrates of different β-galactosidases include ganglioside GM1, lactosylceramides, lactose, and various glycoproteins. == Properties and functions == β-galactosidase is an exoglycosidase which hydrolyzes the β-glycosidic bond formed between a galactose and its organic moiety. It may also cleave fucosides and arabinosides but with much lower efficiency. It is an essential enzyme in the human body. Deficiencies in the protein can result in galactosialidosis or Morquio B syndrome. In ''E. coli'', the gene of β-galactosidase, the ''lacZ'' gene, is present as part of the inducible system ''lac'' operon which is activated in the presence of lactose when glucose level is low. Beta-gal is inhibited by phenylthyl thio-beta-D-galactoside (PETG), L-ribose, D-galactonolactone, and competitive inhibitor isopropyl thio-beta-D-galactoside (IPTG). It is commonly used in molecular biology as a reporter marker to monitor gene expression. It also exhibits a phenomenon called α-complementation which forms the basis for the blue/white screening of recombinant clones. This enzyme can be split in two peptides, LacZα and LacZΩ, neither of which is active by itself but when both are present together, spontaneously reassemble into a functional enzyme. This property is exploited in many cloning vectors where the presence of the ''lacZα'' gene in a plasmid can complement in ''trans'' another mutant gene encoding the LacZΩ in specific laboratory strains of ''E. coli''. However, when DNA fragments are inserted in the vector, the production of LacZα is disrupted, the cells therefore show no β-galactosidase activity. The presence or absence of an active β-galactosidase may be detected by X-gal, which produces a characteristic blue dye when cleaved by β-galactosidase, thereby providing an easy means of distinguishing the presence or absence of cloned product in a plasmid. In 1995, Dimri et al. proposed a new isoform for beta-galactosidase with optimum activity at pH 6.0 (Senescence Associated beta-gal or SA-beta-gal) which would be specifically expressed in senescence (The irreversible growth arrest of cells). Specific quantitative assays were even developed for its detection. However, it is now known that this is due to an overexpression and accumulation of the lysosomal endogenous beta-galactosidase, and its expression is not required for senescence. Nevertheless, it remains the most widely used biomarker for senescent and aging cells, because it is reliable and easy to detect. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beta-galactosidase」の詳細全文を読む スポンサード リンク
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