THROMBIN ACTIVABLE FIBRINOLYSIS INHIBITOR (TAFI)-ROLE IN FIBRINOLYTIC SYSTEM

Presented by: Dr.Gargi TignathGuided by: Dr. Kamal Malukani

INTRODUCTION:

The fibrinolytic system in mammalian blood plays an important role in the dissolution of blood clot & in the maintainence of a patent vascular system.

It contains an inactive proenzyme ,plasminogen which is converted to the active enzyme plasmin.

Plasmin degrades fibrin into soluble fibrin degradation products.

Two immunologically distinct physiologic plasminogen activators in blood are:

(1.)t-PA,tissue –type plasminogen activator.

(2.)u-PA,urokinase –type plasminogen activator.

Inhibition of fibrinolysis occurs at the level of plasminogen activation by-

(1.)specific plasminogen activator inhibitor(PAI-1)

(2.)Thrombin activable fibrinolysis inhibitor(TAFI)

At the level of plasmin,by ᾳ2 antiplasmin.

TAFI, belongs to family of metallocarboxypeptidases .

424AA prepropeptide,22AA signal peptide,92 AA activation peptide,309AA catalytic domain

circulates in plasma as a 60 kDa pro-form upon glycosylation.

Mature CPU protien has 401 AA,with mol.wt of 45.9kDa.

Apparent mol.wt of activeTAFI is 35kDa

Exists In two isoforms,one has threonine at 325 position,other has isoleucine at same position.

TAFI - PROPERTIES

MECHANISM OF ACTION

This Glycoprotein has majority of its N-linked carbohydrates mainly attached to the prodomain, which can be released by proteolysis of the Arg92-Ala93 peptide bond, generating the mature form, TAFIa.

The activity of TAFIa, although

short-lived.

SINGLE NUCLEOTIDE POLYMORPHISMS IN THE TAFI GENE Broad interindividual variation is seen in the plasma TAFI conc.

(upto 10 fold between highest and lowest conc).

A Low or high proCPU plasma conc.tips the balance between profibrinolytic and antifibrinolytic pathways

The human TAFI gene is located on chromosome no.13q14.11 and consists of 11 exons spanning approx 48kb.

Two polymorphisms in the coding sequence were first described-

1.A 505 G>A substitution at AA 147-Leading to Ala- to thr substitution.

2.C678T,substitution –leading to silent polymorphism.

Circulating levels of TAFI are strongly determined by polymorphic variations in the promoter region & the 3‘UTR of TAFI gene.

Regulation of TAFI gene expression is complex ,probably involves more than one single functional variant.

Recently functional studies have demonstrated importance of 3‘UTR polymorphisms in the regulation of TAFI antigen

levels.

INHIBITORS OF TAFI Zinc,is essential requirement for the catalytic action of TAFI.

It is sensitive to inhibition by chelating agents like,EDTA &o-phentroline.

Dithiotheriotol &2-mercaptoethanol-reduces the disulphide linkages in TAFI.

Chloromercurbenzoic acid-inhibits TAFI due to free cysteine ,reused in the enzyme.

Natural carboxypeptidase from potato and leech-PTCI &LCI.

CLINICAL APPLICATIONS & THERAPUTIC CHALLENGES

There is wide interindividual variation in the Plasma TAFI antigen forms.

Recently ,an antigen kit which is genotype independent has been developed,(e.g-independent of ↑ or↓ reactivity towards 325Thr/Ile and 147Ala/Thr polymorphisms).

A low or high pro CPU plasma conc.maintains the balance between profibrinolytic and antifibrinolytic pathway-thereby cause predisposition to bleeding or thrombosis.

Several Studies on conditions like Deep Vein thrombosis & Coronary Artery Diseases with respect to TAFI plasma conc.was found to be inconclusive.

Montaner et al-showed increased levels of TAFI antigen measured with in 24hr in a small samples(30) of elderly pts with ischemic attacks.

Santamari et al measured TAFI activity with in 1st month of ischemic attacks/TIAs,after full activation of TAFI with thrombin thrombinomodulin & found to be ↑ in pts as compare to controls.

Leebeek et al found ↑functional TAFI levels by clot lysis assay in 124 pts with recent ishemic strokes.

SUMMARY TAFI-A basic carboxypeptidase,which inhibits fibrinolysis.

Cleaves C-terminal residues on plasmin-modified partially degraded fibrin.

An increasing no. of studies are finding its role in large no. of prothrombotic disorders like CAD/Strokes.

Antigen levels of TAFI,show stringent association with SNPs,found mostly on regulatory region of TAFI gene.

TAKE HOME MESSAGE

TAFI-is involved in fibrinolysis.

Therefore potential inhibitors of this protien ,makes it an attractive target for future therapeutic practices.