Oreffo V I, John R A, Richards R J
Department of Biochemistry, University of Wales, College of Cardiff, U.K.
Biochem Pharmacol. 1991 Apr 15;41(8):1209-15. doi: 10.1016/0006-2952(91)90660-w.
Lung epithelial type II cells are responsible for synthesising and secreting pulmonary surfactant which reduces surface tension and prevents lung collapse. Type II cells replace type I cells and can proliferate in response to alveolar injury. An important aspect of this proliferation may be the ability of type II cells to accumulate amines actively, particularly the endogenous diamine putrescine. Putrescine is accumulated into isolated alveolar type II cells by an energy-dependent process. The uptake obeys saturation kinetics for which an apparent Km of 14.7 microM and Vmax of 130 pmol/micrograms DNA/hr was derived. The inhibitory effects of structurally similar amines on putrescine accumulation are described. As the herbicide paraquat has been suggested to share the same uptake system as putrescine from lung slice studies, this phenomenon was investigated in type II cell cultures. The results demonstrated that paraquat is a partially competitive inhibitor of putrescine accumulation in the cells. The Ki for the inhibition of putrescine uptake by paraquat in type II cells was calculated to be 69 microM, a value which closely matches the Km for paraquat (70 microM) predicted from lung slice studies. In molecular terms, the partial nature of the competition indicates that paraquat and putrescine do not occupy identical sites. Saturation of its site by paraquat reduced the affinity of putrescine 3.6-fold, but did not abolish it.
肺上皮II型细胞负责合成和分泌肺表面活性物质，该物质可降低表面张力并防止肺塌陷。II型细胞可替代I型细胞，并能在肺泡损伤时增殖。这种增殖的一个重要方面可能是II型细胞主动积累胺类的能力，特别是内源性二胺腐胺。腐胺通过能量依赖过程积累到分离的肺泡II型细胞中。摄取遵循饱和动力学，由此得出的表观Km为14.7微摩尔，Vmax为130皮摩尔/微克DNA/小时。描述了结构相似的胺类对腐胺积累的抑制作用。由于从肺切片研究中表明除草剂百草枯与腐胺具有相同的摄取系统，因此在II型细胞培养中对这一现象进行了研究。结果表明，百草枯是细胞中腐胺积累的部分竞争性抑制剂。计算出百草枯在II型细胞中抑制腐胺摄取的Ki为69微摩尔，该值与从肺切片研究预测的百草枯Km（70微摩尔）非常匹配。从分子角度来看，竞争的部分性质表明百草枯和腐胺并不占据相同的位点。百草枯对其位点的饱和使腐胺的亲和力降低了3.6倍，但并未消除。
