Department of Clinical Biochemistry, Aarhus University Hospital, 8000 Aarhus, Denmark.
J Biol Chem. 2012 Sep 28;287(40):33917-25. doi: 10.1074/jbc.M112.398016. Epub 2012 Aug 7.
Cobalamin uptake and transport in mammals are mediated by three cobalamin-binding proteins: haptocorrin, intrinsic factor, and transcobalamin. The nature of cobalamin-binding proteins in lower vertebrates remains to be elucidated. The aim of this study was to characterize the cobalamin-binding proteins of the rainbow trout (Oncorhynchus mykiss) and to compare their properties with those of the three human cobalamin-binding proteins. High cobalamin-binding capacity was found in trout stomach (210 pmol/g), roe (400 pmol/g), roe fluid (390 nmol/liter), and plasma (2500 nmol/liter). In all cases, it appeared to be the same protein based on analysis of partial sequences and immunological responses. The trout cobalamin-binding protein was purified from roe fluid, sequenced, and further characterized. Like haptocorrin, the trout cobalamin-binding protein was stable at low pH and had a high binding affinity for the cobalamin analog cobinamide. Like haptocorrin and transcobalamin, the trout cobalamin-binding protein was present in plasma and recognized ligands with altered nucleotide moiety. Like intrinsic factors, the trout cobalamin-binding protein was present in the stomach and resisted degradation by trypsin and chymotrypsin. It also resembled intrinsic factor in the composition of conserved residues in the primary cobalamin-binding site in the C terminus. The trout cobalamin-binding protein was glycosylated and displayed spectral properties comparable with those of haptocorrin and intrinsic factor. In conclusion, only one soluble cobalamin-binding protein was identified in the rainbow trout, a protein that structurally behaves like an intermediate between the three human cobalamin-binding proteins.
哺乳动物通过三种钴胺素结合蛋白（即触珠蛋白、内因子和转钴胺素）摄取和转运钴胺素。低等脊椎动物的钴胺素结合蛋白的性质仍有待阐明。本研究旨在阐明虹鳟（Oncorhynchus mykiss）的钴胺素结合蛋白，并将其特性与三种人类钴胺素结合蛋白进行比较。研究发现，虹鳟的胃（210 pmol/g）、鱼卵（400 pmol/g）、鱼卵液（390 nmol/升）和血浆（2500 nmol/升）中均存在高结合能力的钴胺素结合蛋白。基于部分序列分析和免疫反应，所有情况下似乎都是同一种蛋白。该虹鳟的钴胺素结合蛋白从鱼卵液中被分离、测序，并进一步得到了鉴定。与触珠蛋白一样，该虹鳟的钴胺素结合蛋白在低 pH 值下稳定，对钴胺素类似物 cobinamide 具有高结合亲和力。与触珠蛋白和转钴胺素一样，该虹鳟的钴胺素结合蛋白存在于血浆中，并能识别具有改变核苷酸部分的配体。与内因子一样，该虹鳟的钴胺素结合蛋白存在于胃中，可抵抗胰蛋白酶和糜蛋白酶的降解。它在 C 末端的初级钴胺素结合位点的保守残基组成方面也与内因子相似。该虹鳟的钴胺素结合蛋白发生了糖基化，且其光谱特性与触珠蛋白和内因子相当。总之，在虹鳟中仅鉴定出一种可溶性的钴胺素结合蛋白，该蛋白在结构上介于三种人类钴胺素结合蛋白之间。
