Tartrate resistant acid phosphatase

Tartrate resistant acid phosphatase is a glycosylated monomeric metalloenzyme expressed in mammals.^[1] It has a molecular weight of approximately 35kDa, a basic isoelectric point (7.6-9.5) , and optimal activity in acidic conditions. TRAP is synthesized as latent proenzyme and activated by proteolytic cleavage and reduction.^[2][3] It is differentiated by other mammalian acid phosphatases by its resistance to inhibition by tartrate, molecular weights and characteristic purple colour.

The mechanism of phosphate ester hydrolysis by TRAP is through a nucleophilic attack mechanism,^[4] whereby, catalysis occurs with the binding of a phosphate-substrate to the Fe2+ in the active site of TRAP. This is then followed by a nucleophilic attack by a hydroxide ligand on the bound phosphorus atom resulting in cleavage of the phosphate ester bond and production of an alcohol. The exact identity and mechanism of the hydroxide ligand is unclear, but it is thought to be either a hydroxide that bridges the metal ions within the active site, or a terminal hydroxide bound to Fe3+, with conflicting reports for both mechanisms.

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TRAP expression and cell localization

Normally, TRAP is highly expressed by osteoclast, activated macrophages, neurons and the porcine endometrium during pregnancy.^[5][6] In newly born rats, lower levels are also detectable in the spleen, thymus, liver, kidneys, skin, lung and heart. There are also certain pathological conditions whereby expression of TRAP is increased. These include patients with leukaemic reticuloendotheliosis (hairy cell leukaemia), Gaucher’s disease, HIV-induced encephalopathy, osteoclastoma and in osteoporosis and metabolic bone diseases. In osteoclasts, TRAP is localized within the ruffled border area, lysosomes and in Golgi cisternae and vesicles.^[3]

TRAP gene, promoter organisation and transcription

Mammalian TRAP is encoded by one gene, which is localized on chromosome 19 (19p13.2-13.3) in humans, and on chromosome 9 in mice. TRAP DNA is, as expected from protein sequencing, highly conserved throughout the class mammalia. The TRAP gene has been cloned and sequenced in porcine, rat, human and murine species.^[7] Human, murine and porcine TRAP genes all contain 5 exons and have the ATG codon at the beginning of exon 2, with exon 1 being non-coding. Within the exon 1 promoter there are three distinct “tissue-specific” promoters; 1A, 1B and 1C.^[8] This would allow TRAP expression to be tightly controlled. Transcribed from this gene is a 1.5kb mRNA with an open reading frame (ORF) of 969-975 bp encoding a 323-325 amino acid protein. In the rat the ORF is 981bp in length and encodes for a 327 amino acid protein. TRAP is translated as a single polypeptide.

Physiology

The exact physiological role(s) of TRAP is unknown, but many functions have been attributed to this protein. In knockout mice studies, those with a phenotype of TRAP-/- showed mild osteopetrosis, with greatly reduced osteoclast activity, resulting in thickening and shortening of the cortices, the formation of club-like deformities in the distal femur, and widened epiphyseal growth plates with delayed mineralization of cartilage, all of which increased with age.^[9] Likewise in TRAP overexpressing transgenic mice, mild osteoporosis occurred along with increased osteoblast activity and bone synthesis.^[10] Proposed functions of TRAP include osteopontin /bone sialoprotein dephosphorylation, the generation of reactive oxygen species (ROS), iron transport, and as a cell growth and differentiation factor.

Protein dephosphorylation and osteoclast migration

It has been shown that osteopontin and bone sialoprotein, bone matrix phosphoproteins, are highly efficient in vitro TRAP substrates, which bind to osteoclasts when phosphorylated.^[11] Upon partial dephosphorylation, both osteopontin and bone sialoprotein are incapable of binding to osteoclasts. From this effect, it has been hypothesized that TRAP is secreted from the ruffled border, dephosphorylates osteopontin and allows osteoclast migration, and further resorption to occur.

ROS generation

Reactive oxygen species (ROS) are generated in macrophages and osteoclasts from superoxide (O₂^-.), which forms from the action of NADPH-oxidase on oxygen (O₂).^[12] They play an essential role in the function of phagocytic cells.

TRAP, containing a redox active iron, catalyzes the generation of ROS through Fenton chemistry:^[13]

O2 → (NADPH-oxidase) O2- ∙ → (superoxide dismutase) H₂O₂ → (catalase) H₂O + O₂

TRAP-Fe3+ (purple) + O2- ∙→ TRAP-Fe2+ (pink) + O₂

H2O2 + TRAP-Fe2+ (pink) → OH∙ + OH⁻ + TRAP-Fe3+

producing hydroxyl radicals, hydrogen peroxide and singlet oxygen. In osteoclasts, ROS are generated at the ruffled border and seem to be required for resorption and degradation to occur.

Iron transport

In the pregnant sow, uteroferrin is highly expressed in the uterine fluids.^[14] Due to the unique anatomy of the porcine uterus, and the specific, progesterone induced, expression of TRAP; it is hypothesized that uteroferrin acts as an iron transport protein.

Cell growth and differentiation factor

TRAP is associated with osteoblast migration to bone resorption sites, and once there TRAP is believed to initiate osteoblast differentiation, activation and proliferation. This hypothesis was formed from the examination of the bone structure of TRAP-null mice. It was noted that, in addition to osteopetrosis, bone formation occurred in a haphazard manner, where the microarchitecture was highly irregular.^[15]

In TRAP overexpressing mice, it has been found that the affected mice are grossly obese. This has led to the hypothesis that TRAP has involvement in hyperplastic obesity.

References