NPP1,PC1 PC-1(plasma cell membrane glycoprotein)
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Non-lineage Restricted Molecule
Type 2 glycoprotein
Plasma Cell
T Cell, Resting
B Cell, Resting
115 / 120
220 / 240

NPP1 PC-1 is strongly expressed on the surface of plasma cells and is very weakly expressed or absent from resting B and T cells, although high sensitivity techniques may reveal very small amounts on resting B cells.  Expression on lymphoid cells increases with the activation by mitogens and agents that activate protein kinases A and C.  NPP1 is expressed on macrophages and are strongly expressed in bone and cartilage notably on chondrocytes.  Expression also occurs on the epithelium of epididymis, capillary endothelium in the brain, the distal convoluted tubule and collecting ducts of the kidney and on the basolateral surface of hepatocytes.  PC-1 is expressed in non lymphoid tissues including epithelial cells in the testis, salivary gland, kidney, brain capillaries and bone chondrocytes.  NPP1 is located in the endoplasmic reticulum, Golgi and the basolateral plasma membrane when expressed in polarized epithelia.  The enzyme can be released from cells in a water-soluble form, probably by proteolytic cleavage and can be detected in serum and in seminal fluid. 

MOLECULAR FAMILY NAME:  ecto-nucleotide pyrophosphatase/phosphodiesterase family -->ectoenzyme

NPP1 PC-1 names: ENPP1, plasma-cell membrane glycoprotein (PC-1, plasma cell-1), alkaline phosphodiesterase 1, EC (, EC (, nucleotide pyrophosphatase 1 (NPPS, nucleotide pyrophosphatase, NNP1), phosphodiesterase, membrane component chromosome 6 surface marker 1 (M6S1), PCA1, PDNP1, ly-41 (mouse).

NPP1 is a type 2 membrane glycoprotein.  It is a member of a small multigene family of ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP).  It is comprised of 2 disulfide bonded subunits.  The cytoplasmic tail directs the protein to the basolateral surface of epithelial cells.  It is poorly conserved between human and mouse being 76 aa in humans and 58 aa in the mouse.  There are "islands" of conservation within the cytoplasmic tail.  These contain the sequences that determine basolateral expression.  The transmembrane region is predicted to contain 21 aa, none of which are charged.  The transmembrane sequence is nearly identical in human, mouse and rat.  The 85 aa immediately adjacent to the extracellular surface of the membrane are very rich in cysteine and are made up of 2 repeats that may act as a "stalk" to hold the protein away from the membrane.  This region is homologous to a molecule known as "somatomedin B", which was at one time thought to be a growth factor.  The active site threonine is situated at 238 aa in mice and 256 aa in humans.  This threonine is conserved in all family members and is essential for catalytic activity.  Even a change to serine greatly reduces but does not abolish enzyme activity.  There are similarities between 800 aa-811 aa  and to the calcium-binding EF hand motifs seen in calmodulin and other calcium-binding proteins, but the evidence that they bind calcium is weak and their 3-dimensional arrangement is not yet known.

PC-1 is a disulfide linked homodimeric type 2 glycoprotein.  The membrane-proximal region is comprised of 2 somatomedin-B like domains, followed by a catalytic domain characteristic of 5' nucleotidases.  These 2 consensus sequences of EF-hand like divalent cation binding motifs which are found between residues 265-294 and 739-767.

Cell Type Unreduced Reduced Comment
various 240 kDa 120 kDa NPP1 is a type II membrane protein, with its amino terminus inside the cell, a single transmembrane/noncleavable signal surface, and a large extracellular domain that contains the catalytic site and one or possibly two EF hand calcium-binding motifs.  It is a disulfide-bonded homodimer, with each chain of molecular weight about 120,000

Cell type Unreduced Reduced Comment
220 kDa 115-120 kDa PC-1

ENPP2 and ENPP3 have similar structures, while the newly described putative NNP4 and NPP5 are shorter and appear to be type 1 membrane proteins.



The human NPP1 PC-1, has 8 potential sites for N-linked glycosylation and has been shown to be glycosylated in vivo.  The mouse NPP1 PC-1, has 6 potential sites for N-linked glycosylation and most or all are glycosylated.  There is no information concerning O-linked glycosylation.  The active site threonine is situated at 238 aa in the mouse and at 256 aa in the human and is transiently adenylated and/or phosphorylated during the catalytic cycle.

Molecule Comment
insulin receptor (IR) A physical association with the insulin receptor has been reported

NPP1 expression is not essential for production of viable mice and does not seem to be essential for normal cell survival.  NPP1 PC-1 is only expressed on certain highly specialized cell types such as plasma cells, chondrocytes, hepatocytes, distal convoluted tubule and collecting ducts of the kidney, brain capillaries, salivary ducts and epithelium of epididymis.  It is unlikely to have general cellular function.  Possible roles for NPP1 include regulation of pyrophosphate production, regulation of the availability of nucleotide sugars in the endoplasmic reticulum and Golgi and regulation purinergic signaling. 

PC-1 is an ecto-enzyme with alkaline phosphodiesterase I, EC, and nucleotide pyrophosphatase, EC, activities.  PC-1 was also found to have auto-phosphorylation activity, and auto-phosphorylation inactivates its other 2 enzymatic activities.  It has been suggested that auto-phosphorylation at a low level ATP concentration is a regulatory mechanism which prevents depletion of nucleotides when they are scarce.


Ecto-nucleotide pyrophosphatase/phosphodiesterase NPP1 PC-1 accounts for nearly all the ecto-pyrophosphatase activity of chondrocytes.  NPP1 PC-1 is capable of auto-adenylation, auto-phosphorylation and auto-dephosphorylation.  NNP1 PC-1 has been claimed to be capable of phosphorylating other proteins but the evidence is weak.


NPP1 PC-1 plays an important role in normal and pathological calcification of bone and cartilage.  Deletion of the NPP1 PC-1 gene by genetic engineering and also in the naturally occurring mutant tip toe walking mice causes extensive calcification in and around joints and ligaments.  In humans, a polymorphism of the PC-1 gene involving the substitution of glutamine for lysine at position 173, K173Q and was previously K121 due to incorrect assignment of the initiator methionine, has been associated with insulin resistance in type 2 diabetes in a population of Caucasians in Sicily.  Mutations in human ENPP1 have been correlated with ossification of the posterior longitudinal ligament (OPLL) of the spine.  In neither case has a casual link between the polymorphisms and disease been proven.  PC-1 over-expression has been implicated as a primary cause of insulin resistance in type 2 diabetes mellitus although this remains controversial.


NPP1 PC-1 has been implicated in the regulation in the calcification of tissues by controlling levels of extracellular pyrophosphate and may also play a role in regulating purinergic signalling by destroying extracellular ATP or creating extracellular AMP which can be converted to adenosine by 5' nucleotidase.  ATP can act on P2 purinergic receptors.  Adenosine can act on P1 purinergic receptors.



NPP1 PC-1has a broad specificity and cleaves a variety of substrates including phosphodiester bonds of nucleotides and nucleotide sugars and including pyrophosphate bonds of nucleotides and nucleotide sugars.  NPP1 PC-1 can hydrolyze nucleoside 5' triphosphates such as ATP, GTP, CTP, TTP and UTP to their corresponding monophosphates with the release of pyrophosphate.  NNP1 PC-1 can hydrolyze diadenosine polyphosphates and 3', 5'-cAMP to AMP.  The concomitant release of pyrophosphate may inhibit formation of hydroxyapatite crystals.  Absence of the enzyme results in excessive calcification in and around joints.  The enzymic activity of NPP1 PC-1 is inhibited by the P2 puirinergic inhibitors reactive blue (RB2) and pyrodoxalphosphate-6-azaphenyl-2'.4'-disulphonic acid (PPADS), which are available from Sigma Chemical Company, Missouri, USA.

Enzyme Comment
Glycosyl transferases


NPP1 has sometimes been referred to as PCA1 and PCA-1 but these terms are misleading and should be discontinued because they have also been used for different and unrelated molecules.  PC-1 stands for "plasma cell-1" and should not be confused with pro-hormone convertase, a protease that has also been named PC-1 but which is totally unrelated in structure or in function.  The previous HUGO name was PDNP1.  NPP1 was initially named because it seemed to be specific for plasma cells.  Subsequent work showed that it is present on a number of other cell types.  Even within the immune system, it cannot be regarded as totally specific for plasma cells.  The best evidence for its selective expression on plasma cells is in the mouse and the question of whether it is a useful marker of plasma cells in humans remains unanswered at the present time. 

The ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) or the PC-1 family has at least 5 members named in order of discovery.  The 1st 3 members, ENPP1-3, are type 2 transmembrane proteins with their amino-terminus in the cytoplasm, while ENNP4 and ENPP5 are type 1 membrane proteins with cleavable N-terminal signal sequences and a putative transmembrane domain close to the C-terminus, and a carboxy-terminal cytoplasmic tail.  The 1st family member is the plasma cell antigen PC-1 and is also known as PC.1, PCA1, Pca-1, ecto-nucleotide pyrophosphatase/phosphodiesterase I, NPP1, PDNP1, M6S1, NPPg and NPPS.  In order to conform to their nomenclature rules, the Human Gene Nomenclature Committee designated this gene ENPP1 in February 2000.  The 2nd family member is known as autotaxin and is also known as ATX, PD-1a, PDIa, NPPa  and PDNP2.  The committee designated this gene ENPP2 in February 2000.  The 3rd family member is known as B10 and is also known as gp130RB13-6, bovine intestinal phosphodiesterase, PD-1b , PDNP3.  The committee designated this gene ENPP3 in February 2000.  Recently 2 more putative ENPP genes have been identified and tentatively named ENPP4 and ENPP5.  The ecto-nucleotide pyrophosphatase/phosphodiesterases are related in structure and in catalytic activity to alkaline phosphatase, arylsulfatases and cofactor-independent phosphoglycerate mutase. 

The enzymic activity of NPP1 PC-1 and related proteins involves hydrolysis of nucleotides and nucleotide sugars with the release of pyrophosphate and/or inorganic phosphate.  These activities have the potential to influence calcification of bone and cartilage and also to influence signaling via purinergic receptors by destruction of extracellular ATP and generation of ADP and AMP and related molecules.  The enzyme family has been implicated in diverse biological activities including osteoarthritis, OPLL of the spine, insulin resistance in type 2 diabetes and tumor cell motility.

Database accession numbers

Revised June 25, 2008

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