Hao Z, Reiske H R, Wilson D B
Institute for Comparative and Environmental Toxicology, Cornell University, Ithaca, New York 14853, USA.
Appl Environ Microbiol. 1999 Nov;65(11):4741-5. doi: 10.1128/AEM.65.11.4741-4745.1999.
Two different Cd(2+) uptake systems were identified in Lactobacillus plantarum. One is a high-affinity, high-velocity Mn(2+) uptake system which also takes up Cd(2+) and is induced by Mn(2+) starvation. The calculated K(m) and V(max) are 0.26 microM and 3.6 micromol g of dry cell(-1) min(-1), respectively. Unlike Mn(2+) uptake, which is facilitated by citrate and related tricarboxylic acids, Cd(2+) uptake is weakly inhibited by citrate. Cd(2+) and Mn(2+) are competitive inhibitors of each other, and the affinity of the system for Cd(2+) is higher than that for Mn(2+). The other Cd(2+) uptake system is expressed in Mn(2+)-sufficient cells, and no K(m) can be calculated for it because uptake is nonsaturable. Mn(2+) does not compete for transport through this system, nor does any other tested cation, i.e., Zn(2+), Cu(2+), Co(2+), Mg(2+), Ca(2+), Fe(2+), or Ni(2+). Both systems require energy, since uncouplers completely inhibit their activities. Two Mn(2+)-dependent L. plantarum mutants were isolated by chemical mutagenesis and ampicillin enrichment. They required more than 5,000 times as much Mn(2+) for growth as the parental strain. Mn(2+) starvation-induced Cd(2+) uptake in both mutants was less than 5% the wild-type rate. The low level of long-term Mn(2+) or Cd(2+) accumulation by the mutant strains also shows that the mutations eliminate the high-affinity Mn(2+) and Cd(2+) uptake system.
在植物乳杆菌中鉴定出两种不同的镉（Cd²⁺）摄取系统。一种是高亲和力、高速度的锰（Mn²⁺）摄取系统，该系统也能摄取Cd²⁺，并在Mn²⁺饥饿时被诱导。计算得出的米氏常数（Kₘ）和最大反应速度（Vₘₐₓ）分别为0.26微摩尔和3.6微摩尔每克干细胞每分钟。与由柠檬酸盐和相关三羧酸促进的Mn²⁺摄取不同，Cd²⁺摄取受到柠檬酸盐的微弱抑制。Cd²⁺和Mn²⁺是彼此的竞争性抑制剂，并且该系统对Cd²⁺的亲和力高于对Mn²⁺的亲和力。另一种Cd²⁺摄取系统在Mn²⁺充足的细胞中表达，由于摄取不饱和，因此无法计算其Kₘ。Mn²⁺不通过该系统竞争运输，其他任何测试的阳离子，即锌（Zn²⁺）、铜（Cu²⁺）、钴（Co²⁺）、镁（Mg²⁺）、钙（Ca²⁺）、铁（Fe²⁺）或镍（Ni²⁺）也不会。两种系统都需要能量，因为解偶联剂会完全抑制它们的活性。通过化学诱变和氨苄青霉素富集分离出两个依赖Mn²⁺的植物乳杆菌突变体。它们生长所需的Mn²⁺比亲本菌株多5000倍以上。两个突变体中Mn²⁺饥饿诱导的Cd²⁺摄取均低于野生型速率的5%。突变菌株长期积累的低水平Mn²⁺或Cd²⁺也表明，这些突变消除了高亲和力的Mn²⁺和Cd²⁺摄取系统。
