Tryptase

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Since the 1970s, people have used trypsin to stain mast cells (MC) through enzyme tissue staining, and found that MC can be stained, indicating that MC must contain trypsin active substances. In 1981, Schwartz et al. further purified this enzyme and found that it was released by MC, and more than 90% of its activity came from an enzyme, so it was named tryptase. Miller et al. cloned the first type of trypsin cDNA in 1989, and later several trypsin-like proteases were cloned. Tryptase is divided into three categories at the cDNA and protein levels: α, β, and γ, with β content being the highest. Each tryptase gene contains 6 exons and 5 introns, encoding a 30 amino acid leader chain and 245 amino acid active site. Inferred from the amino acid sequence, α-trypsin and β-trypsin have 90% homology. The main difference is that the -3 and 215 amino acids of β-trypsin are arginine and glycine respectively, while the α-trypsin is glutamine and aspartic acid, respectively. The structural difference between the two determines they differ in activity.

Figure 1. Protein structure of tryptase.

Structure

Pereira et al. clarified that the crystal structure of human β-tryptase is a cyclic homotetramer, and its crystal is composed of four monomers. The four monomers have an active center on each of the four corners pointing to a relatively narrow central hole (50A×30A), which prevents the effects of multiple inhibitors. α-Trypsin is an inactive tetrameric structure, only a monomer form. β-trypsin and γ-trypsin are similar in structure, with only one less c-terminal hydrophobic tail.

The role of tryptase in promoting angiogenesis and tumor

It is recognized that angiogenesis is closely related to tumor growth. Experiments show that MC contains angiopoietin, which and tryptase can induce the proliferation of vascular endothelial cells. Tryptase can degrade the connective tissue matrix and provide enough space for tumor blood vessels to grow, thereby promoting the occurrence and development of tumors. Someone found that patients with six vascular diseases such as purulent granuloma, malignant hemangioendothelioma, cherry-like hemangioma, pigmented nevus, sponge-like hemangioma, and Kapodi's sarcoma were stained with tryptase and found that the density of MC was significantly higher than that of normal tissues.

Tryptase and allergic diseases

MC is the main effector cell of the body's rapid allergic reaction. When the body is sensitized, MC releases β-trypsin in the blood, and its concentration in the blood reaches its peak after 1h, with a half-life of 2h. This allows the body to detect β-trypsin even after hours of sensitization. Through the ELISA method, it is found that only α-trypsin can be detected in the blood of normal people, while β-trypsin is almost undetectable. However, testing the bronchial lavage fluid of patients with asthma and allergic rhinitis, tears of patients with allergic ophthalmia, joint synovial fluid of patients with rheumatism, and serum of patients with systemic allergic reactions found that the content of β-trypsin was significantly increased.

Tryptase and mastocytosis

Mastocytosis is diagnosed mainly by bone marrow biopsy to find abnormally proliferating spindle-shaped MC. The experiment found that it is mainly related to the amount of α-trypsin. While α-trypsin is spontaneously secreted, not affected by the status of MC, but it can reflect the number of MC. It is clinically found that the bone marrow biopsy is 100% positive when α-trypsin is >75ng/ml, and it is more sensitive than the biopsy at 20~75ng/ml. Therefore, α-trypsin has a good diagnostic value for mastocytosis.

Clinical use

Serum levels are normally less than 11.5 ng/mL. Elevated levels of serum tryptase occur in both anaphylactic and anaphylactoid reactions, but a negative test does not exclude anaphylaxis. Tryptase is less likely to be elevated in food allergy reactions as opposed to other causes of anaphylaxis. Serum tryptase levels are also elevated in and used as one indication suggesting the presence of eosinophilic leukemias due to genetic mutations resulting in the formation of FIP1L1-PDGFRA fusion genes or the presence of systemic mastocytosis