Ceruloplasmin (CP; UniProt P00450) is a multicopper glycoprotein with a principal ferroxidase activity. Oxidizing Fe(II) to Fe(III) links the enzyme directly to iron handling and cellular iron homeostasis. The authored core location describes directly studied human soluble extracellular CP. Mammalian membrane-associated CP is a distinct product context: the GO extracellular-region definition excludes material attached to the cell surface. The plasma-membrane IBA is retained with its phylogenetic and mammalian evidence, but the rat construct experiment and an ER-retained mutant do not establish a particular normal human membrane product. This narrower core summary does not assert that human GPI-linked CP is absent.
The source evidence provides the source identities, current ontology definitions, exact primary locators, sequence and phylogenetic projections, donor records and source-access limitations supporting the individual decisions.
The official human displayed sequence is a 1,065-residue precursor, with signal peptide 1–19 and mature chain 20–1065, linked to RefSeq NP_000087.2. The current UniProt entry has no ALTPROD list. That absence does not establish that only one biological product exists. No alternate human sequence or accession is inferred from a different species' constructs.
The original alternative-splicing study, PMID:10660599 used rat C6 glioma and liver cDNAs, rat-specific antibody and rat tissue RNA, with constructs expressed in human HEK293T cells. Its last-five-residue replacement and reported product lengths belong to those rat constructs. The publicly indexed Experimental Procedures and Results support GPI-sensitive surface localization in that system, not a reconstructed human P00450 isoform. Separately, PMID:2355023(https://www.researchgate.net/publication/20801112_Human_ceruloplasmin_Tissue-specific_expression_of_transcripts_produced_by_alternative_splicing) describes a human 12-nucleotide insertion encoding GEYP and tissue-dependent transcripts. It does not establish a complete alternative protein sequence with distinct molecular activity. The original text is available through public indexing even though neither PDF was downloaded. In PMID:10660599, Experimental Procedures, Transfections (printed p.4306), the secreted construct is described as PMID:10660599, "was amplified from rat liver RNA". In PMID:2355023, Results (printed p.10782), the partial clone PMID:2355023, "accounts for only 76% of the full length CP mRNA". The source evidence distinguishes the available indexed Methods/Results from unchanged abstract-only caches and records the specific public sections.
PMID:29183916, "rescue of ferroxidase activity" tested purified human soluble CP administered to Cp-knockout mice. Restored brain ferroxidase activity and reduced iron deposition support performed iron chemistry and iron homeostasis, rather than necessity alone. The experiment does not demonstrate universal blood–brain barrier entry or full equivalence of soluble and membrane-associated products. The HepG2 uptake study PMID:9445478(https://pubmed.ncbi.nlm.nih.gov/9445478/) remains relevant: the evidence is not rewritten as exclusively iron efflux. Established ferroxidase assertions whose older full articles remain unavailable are retained with explicit curator deference and access limits.
The available original PMID:17242517(https://doi.org/10.1107/S090744490604947X) includes human CP purification, crystallization and refinement Methods and copper-site Results. The 2.8 Å structure supports copper binding and distinguishes the mononuclear sites from the trinuclear cluster. The role of the domain-2 copper site is not assumed to equal that of every other site. No figure-pixel inspection is claimed.
PMID:10481051(https://pubmed.ncbi.nlm.nih.gov/10481051/) explicitly reports glutathione-dependent reduction of hydrogen peroxide and lipid hydroperoxides by CP. Those source-specific activities remain positive but secondary to the ferroxidase core. PMID:10508415(https://pubmed.ncbi.nlm.nih.gov/10508415/) reports human CP alkyl-hydroperoxide reduction and Cys699 chemistry, but the available abstract does not resolve the exact hydrogen-peroxide and phospholipid substrates attached to its two existing IDA annotations. Those two assertions remain UNDECIDED; the positive chemistry is preserved. Cuprous-ion oxidation in PMID:14623105(https://pubmed.ncbi.nlm.nih.gov/14623105/) supports the broader metal-ion oxidoreductase category beyond Fe(II), while PMID:16906150(https://pubmed.ncbi.nlm.nih.gov/16906150/) and PMID:4643313(https://pubmed.ncbi.nlm.nih.gov/4643313/) provide additional substrate context without creating another primary core.
The full original PMID:23306200, "binding of ceruloplasmin to myeloperoxidase inhibits the halogenation activity" directly tests CP association with and inhibition of myeloperoxidase. Human plasma protein purification, ELISA, size-exclusion, stopped-flow kinetics and protein controls distinguish reversible enzyme inhibition from simply consuming released oxidants. The mouse-plasma comparison is a separate experiment. The source characterizes this as a minor function relative to ferroxidase activity. PMID:23843990(https://pubmed.ncbi.nlm.nih.gov/23843990/) further examines purified human CP, leukocyte myeloperoxidase and milk lactoferrin complexes by crystallography and SAXS. Measured association and peptide inhibition are retained; proposed physiological inflammatory consequences are not promoted to new process annotations.
In the original clusterin study, PMID:19996109, human plasma underwent prolonged shear and temperature stress. Anti-clusterin affinity purification, mass spectrometry, immunoblotting, size-exclusion and sandwich ELISA identified CP in the complexes. The CP fragments and stress conditions limit claims about intact products in native plasma. Clusterin is an established extracellular folding chaperone. The GO:0051087 definition describes binding to such a protein, including its interaction with unfolded clients; it does not require the client to perform chaperone activity. The controlled association therefore remains non-core client binding, not a CP folding function or a protein-folding process assertion. Original Results, Figure 6, state that PMID:19996109, "complexes were present in stressed but not control plasma". No current IntAct/MINT pair was recovered for this paper; that export limitation does not negate the actual primary experiment.
The actual PMID:20551380 Table I identifies CERU_HUMAN in three human aortic NaCl extracts: four unique peptides, four unique spectra, 2.3% sequence coverage and nine total spectra. These are the NaCl fractions, not the separately studied SDS or guanidine fractions. The result supports extracellular detection, not an independent structural-matrix function.
The urinary exosome paper, PMID:19056867 and its NHLBI protein table identify CP/NP_000087 with six peptides and reference 2. The original isolation and mass-spectrometry filtering Methods were inspected. Its rules permit shared splice-product peptides to support a gene-level entry, so this is not isoform-exclusive evidence.
The available extracellular-vesicle and plasma abstracts support plausible extracellular detection with explicit curator deference where an exact target supplement could not be independently inspected. The source-specific lysosomal-membrane, expressed-prostatic-secretion-vesicle and plasma-microparticle annotations remain UNDECIDED where their target rows were unavailable. Invalid HTML/XML responses were not treated as spreadsheets or negative target results. The copper-homeostasis annotation from PMID:14623105(https://pubmed.ncbi.nlm.nih.gov/14623105/) likewise remains unresolved at the experimental process scope despite positive copper binding and oxidation evidence.
Current Reactome paths distinguish CP's location from that of a larger complex. The CP component R-HSA-904819 is recorded as extracellular within the membrane-associated SLC40A1–CP complex R-HSA-904825. Target paths were reconstructed through reaction catalysts, complexes, sets and reference entities, including the mutant CP set and Fam20C substrate sets. ER annotations describe secretory transit and processing; CP's presence as a phosphorylation substrate does not assign kinase activity to CP. The traversal is target-directed, not a claim to have read every branch of the global pathway graph.
The current PANTHER PTHR11709 tree contains 4,210 nodes and places human CP at PTN002542383 within the ancestry of both cited PAINT nodes, PTN000194698 and PTN000194708. The current alignment has the same 1,065-residue length but D544 rather than E544 in the official human sequence. This difference is documented without inferring loss of activity. Current placement supports the broad inherited redox premise; direct human evidence supports refinement to ferroxidase activity. A short donor list, or the target's own experimental evidence appearing among descendants, is not a reason to reject a PAINT assertion.
The literal donor reconstruction contains 33 source joins and 69 matching record occurrences. Evidence codes and current source references are retained, including cases where a present donor record is electronic rather than direct experimental evidence. Current records do not recreate every historical transfer event.
Official CP identity lists five FunFam signatures. With those signatures included, current ARBA00027769 branch 7 matches both 2.60.40.420:FF:000009 and 2.60.40.420:FF:000015 plus Primates (NCBITaxon:9443). This supports the broad metal-ion oxidoreductase premise; ferroxidase is the more informative principal activity. None of the 280 current ARBA00027128 branches matches the present target signatures and taxonomy. That current mismatch leaves historical rule applicability unresolved, rather than disproving independently supported soluble secretion.
The principal unresolved product question is the exact human membrane-associated sequence and its correspondence to independently measured activities, not whether alternate products are biologically possible. Other open questions concern physiological substrate preference for the secondary peroxide chemistry, the exact target evidence behind several high-throughput compartment assignments, and how CP's secondary myeloperoxidase interaction contributes in vivo relative to iron oxidation. No new GO annotation is inferred from missing GO-CAM coverage or from these questions alone.