Levi S, Santambrogio P, Cozzi A, Rovida E, Corsi B, Tamborini E, Spada S, Albertini A, Arosio P
Department of Biomedical Science and Technology, University of Milano, Italy.
J Mol Biol. 1994 May 20;238(5):649-54. doi: 10.1006/jmbi.1994.1325.
Mammalian ferritins are 24-meric proteins composed of variable proportions of H and L-subunits. The L-chain, in contrast to the H-chain, lacks detectable ferroxidase activity, and its role in ferritin iron incorporation is unclear. In this study, apoferritins were subjected to iron loading with large iron increments to favour spontaneous iron hydrolysis. The homopolymers of the wild-type H-chain, and of a mutant H-chain with an inactivated ferroxidase centre, formed massive protein aggregates, while the L-chain homopolymers remained mostly soluble. The difference between H and L-ferritins was not related to the rate of iron oxidation or to the presence of preformed iron cores. Heteropolymers were constructed in vitro by co-renaturing different proportions of the H-chain with the L-chain or mutant H-chain with an inactivated ferroxidase centre. After loading with high iron increments, protein aggregation of the heteropolymers was reduced when the L-chain content was above 70 to 80%, either in combination with the wild-type H-chain or with the inactivated mutant H-chain. Under acidic conditions (pH 5.5, 1000 Fe atoms per molecule) the heteropolymers with about 20% H and 80% L-chains incorporated three to fourfold more iron into soluble 24-mers than the homopolymers. The data indicate that ferritins with more than 18 L-chains per molecule have the capacity to lower non-specific iron hydrolysis in bulk solution. This property is possibly due to a specific attraction of the incoming oxidized iron into the cavity and may be related to an effect of the L-chain on the cavity microenvironment. It is concluded that under high iron increments the ferritins with high L:H-chain ratios are the most efficient in incorporating iron, and this goes some way to explain why iron storage tissues contain L-rich isoferritins.
哺乳动物铁蛋白是由不同比例的H亚基和L亚基组成的24聚体蛋白。与H链相比，L链缺乏可检测到的铁氧化酶活性，其在铁蛋白铁掺入中的作用尚不清楚。在本研究中，脱铁铁蛋白用大量增加的铁进行负载，以促进铁的自发水解。野生型H链和具有失活铁氧化酶中心的突变H链的同聚物形成大量蛋白质聚集体，而L链同聚物大多保持可溶。H型和L型铁蛋白之间的差异与铁氧化速率或预先形成的铁芯的存在无关。通过将不同比例的H链与L链或具有失活铁氧化酶中心的突变H链共同复性，在体外构建杂聚物。在负载大量增加的铁后，当L链含量高于70%至80%时，无论是与野生型H链还是与失活的突变H链组合，杂聚物的蛋白质聚集都会减少。在酸性条件下（pH 5.5，每分子1000个铁原子），含有约20% H链和80% L链的杂聚物比同聚物多三到四倍地将铁掺入可溶性24聚体中。数据表明，每分子含有超过18条L链的铁蛋白有能力降低大量溶液中的非特异性铁水解。这种特性可能是由于进入的氧化态铁被特异性吸引到腔内，并且可能与L链对腔微环境的影响有关。得出的结论是，在铁大量增加的情况下，具有高L:H链比的铁蛋白在掺入铁方面最有效，这在一定程度上解释了为什么铁储存组织含有富含L链的异铁蛋白。
