Resnick A D, Magasanik B
J Biol Chem. 1976 May 10;251(9):2722-8.
An L-asparaginase has been purified some 250-fold from extracts of Klebsiella aerogenes to near homogeneity. The enzyme has a molecular weight of 141,000 as measured by gel filtration and appears to consist of four subunits of molecular weight 37,000. The enzyme has high affinity for L-asparagine, with a Km below 10(-5) M, and hydrolyzes glutamine at a 20-fold lower rate, with a Km of 10(-3) M. Interestingly, the enzyme exhibits marked gamma-glutamyltransferase activity but comparatively little beta-aspartyl-transferase activity. A mutant strain lacking this asparaginase has been isolated and grows at 1/2 to 1/3 the rate of the parent strain when asparagine is provided in the medium as the sole source of nitrogen. This strain grows as well as the wild type when the medium is supplemented with histidine or ammonia. Glutamine synthetase activates the formation of L-asparaginase. Mutants lacking glutamine synthetase fail to produce the asparaginase, and mutants with a high constitutive level of glutamine synthetase also contain the asparaginase at a high level. Thus, the formation of asparaginase is regulated in parallel with that of other enzymes capable of supplying the cell with ammonia or glutamate, such as histidase and proline oxidase. Formation of the asparaginase does not require induction by asparaginase and is not subject to catabolite repression.
已从产气克雷伯菌提取物中纯化出一种L-天冬酰胺酶，纯化倍数约为250倍，接近均一状态。通过凝胶过滤法测得该酶分子量为141,000，似乎由四个分子量为37,000的亚基组成。该酶对L-天冬酰胺具有高亲和力，Km低于10^(-5)M，对谷氨酰胺的水解速率低20倍，Km为10^(-3)M。有趣的是，该酶表现出显著的γ-谷氨酰转移酶活性，但β-天冬氨酰转移酶活性相对较低。已分离出一种缺乏这种天冬酰胺酶的突变菌株，当培养基中仅提供天冬酰胺作为唯一氮源时，其生长速度为亲本菌株的1/2至1/3。当培养基添加组氨酸或氨时，该菌株生长与野生型一样好。谷氨酰胺合成酶激活L-天冬酰胺酶的形成。缺乏谷氨酰胺合成酶的突变体无法产生天冬酰胺酶，而谷氨酰胺合成酶组成型水平高的突变体也高水平含有天冬酰胺酶。因此，天冬酰胺酶的形成与其他能够为细胞提供氨或谷氨酸的酶（如组氨酸酶和脯氨酸氧化酶）的形成受到平行调节。天冬酰胺酶的形成不需要天冬酰胺酶诱导，也不受分解代谢物阻遏。
