beta-Galactosidases in Ripening Tomatoes.
beta-Galactosidases in Ripening Tomatoes.
复制标题
成熟番茄中的β-半乳糖苷酶。
作者：
R. Pressey
番茄 (Lycopersicon esculentum L.) 含有高水平的 β-半乳糖苷酶活性，这是由于该酶的三种形式所致。在番茄成熟过程中，它们的活性总和保持相对恒定，但各个形式的β-半乳糖苷酶的水平发生显着变化。通过 DEAE-Sephadex A-50 和 Sephadex G-100 的组合色谱分离这三种酶。在番茄成熟过程中，β-半乳糖苷酶 I 和 III 的水平下降，但 β-半乳糖苷酶 II 的水平增加了 3 倍以上。这三种酶在 pH 值接近 4 时具有最佳活性，并且均被半乳糖和半乳糖内酯抑制。然而，这些酶在分子量、与对硝基苯基-β-半乳糖苷的 K(m) 值以及 pH 和温度稳定性方面有所不同。 β-半乳糖苷酶 II 是唯一能够水解从番茄中分离出来的多糖的酶，该多糖主要由 β-1、4 连接的半乳糖组成。 β-半乳糖苷酶 II 降解半乳聚糖的能力及其在番茄成熟过程中活性的增加表明该酶在番茄软化过程中可能发挥作用。
Tomatoes (Lycopersicon esculentum L.) contained a high level of beta-galactosidase activity which was due to three forms of the enzyme. During tomato ripening, the sum of their activities remained relatively constant, but the levels of the individual forms of beta-galactosidase changed markedly. The three enzymes were separated by a combination of chromatography of DEAE-Sephadex A-50 and Sephadex G-100. During ripening of tomatoes, beta-galactosidases I and III levels decreased but the beta-galactosidase II level increased more than 3-fold. The three enzymes were optimally active near pH 4, and all were inhibited by galactose and galactonolactone. However, the enzymes differed in molecular weight, K(m) value with p-nitrophenyl-beta-galactoside, and stability with respect to pH and temperature. beta-Galactosidase II was the only enzyme capable of hydrolyzing a polysaccharide that was isolated from tomatoes and that consisted primarily of beta-1, 4-linked galactose. The ability of beta-galactosidase II to degrade the galactan and the increase in its activity during tomato ripening suggest a possible role for this enzyme in tomato softening.