TY - JOUR
T1 - The role of the L-chain in ferritin iron incorporation. Studies of homo and heteropolymers
AU - Levi, S.
AU - Santambrogio, P.
AU - Cozzi, A.
AU - Rovida, E.
AU - Corsi, B.
AU - Tamborini, E.
AU - Spada, S.
AU - Albertini, A.
AU - Arosio, P.
PY - 1994
Y1 - 1994
N2 - 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 l;he 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.
AB - 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 l;he 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.
KW - Iron mineralization
KW - Iron proteins
KW - Isoferritins
KW - Recombinant proteins
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U2 - 10.1006/jmbi.1994.1325
DO - 10.1006/jmbi.1994.1325
M3 - Article
C2 - 8182740
AN - SCOPUS:0028276960
VL - 238
SP - 649
EP - 654
JO - Journal of Molecular Biology
JF - Journal of Molecular Biology
SN - 0022-2836
IS - 5
ER -