Tissue-nonspecific alkaline phosphatase (TNAP) (TNAP)

Tissue-nonspecific alkaline phosphatase (TNAP) is a homodimeric, glycosylphosphatidylinositol-anchored ectoenzyme that is ubiquitously expressed, with particularly high concentrations in the liver, bone, and kidney [1, 6]. It plays a pivotal role in skeletal mineralization by hydrolyzing extracellular inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal formation, into inorganic phosphate [2, 11]. Additionally, TNAP is crucial for the metabolism of pyridoxal 5'-phosphate (PLP), the active form of vitamin B6, which is necessary for the synthesis of various neurotransmitters in the brain [8, 13]. Loss-of-function mutations in the ALPL gene encoding TNAP lead to hypophosphatasia, a rare genetic disorder characterized by defective bone mineralization, rickets, and seizures [1, 16]. In contrast, elevated TNAP activity is associated with pathological vascular calcification, making it a therapeutic target for chronic kidney disease and atherosclerosis [4, 12]. Currently, TNAP is therapeutically addressed through enzyme replacement therapy with Asfotase alfa for hypophosphatasia, while small-molecule inhibitors are under clinical investigation to treat ectopic calcification [14].

Other names

Alkaline phosphatase, liver/bone/kidney isozymeTNSALPAP-TNAPALPLAlkaline phosphatase 2Liver/bone/kidney-type alkaline phosphatase

Drugs targeting this enzyme work either as replacement therapy to restore physiological substrate hydrolysis or as inhibitors to prevent pathological mineralization [2, 11, 14]. Asfotase alfa is a recombinant fusion protein that replaces deficient TNAP activity to hydrolyze inorganic pyrophosphate (PPi), an inhibitor of hydroxyapatite formation, thereby facilitating bone mineralization in patients with hypophosphatasia [2]. Conversely, small-molecule inhibitors like DS-1211 are designed to reduce TNAP activity in conditions of overexpression, such as vascular calcification, thereby increasing local PPi levels to prevent ectopic mineral deposition in blood vessels [11, 14].