Tissue plasminogen activator (t-PA) exists in two forms: as single-chain (sct-PA) and as two-chain t-PA (tct-PA), the latter form being generated from sct-PA through proteolytic cleavage by plasmin.
They both activate plasminogen to plasmin (plasminogenolytic activity; refer to the Coaset t-PA kit).
Also they both hydrolyse chromogenic substrates (amidolytic activity; refer to the Research Method for t-PA in purified preparations).
A quantitative analysis for the composition of a mixture of one-chain and two-chain t-PA by amidolytic assay has been proposed (1) .
The first International Standard for t-PA, 83/517, was established in 1985 (2) by using a melanoma extract. The potency assignment was done with a fibrin clot lysis method.
In 1987, the second International Standard for t-PA, 86/670, was established (3).
It was again a purified preparation from a cultured melanoma cell supernatant containing about 98% of single-chain t-PA.
Chromogenix t-PA reagent (Art. No. 821157), is composed mainly of single-chain t-PA (>95%) with a specific fibrinolytic activity of about 500,000 IU/mg of enzyme (batch-specific reagent) assessed against the second international standard. The enzyme activity in terms of nkat with the substrate S-2288 is 0.087 nkat with 400 IU (see kinetic tables).

1. Verheijen JH et al.
   Quantitative analysis of the composition of mixtures of one-chain and two-chains tissue-type plasminogen activator with a spectrophotometric method.
   Thromb Res. 39, 281-288 (1985).
2. Gaffney PJ et al.
   A collaborative study of a proposed international standard for tissue plasminogen activator (t-PA).
   Thromb Haemost 53, 134-136 (1985).
3. Gaffney PJ et al. A collaborative study to establish the 2nd international standard for tissue plasminogen activator (t-PA).
   Thromb Haemost 58, 1085-1087 (1987).