Elimination of Native and Carbohydrate-Modified Tissue Plasminogen Activator in Rabbits

Summary

The elimination of native and carbohydrate-modified tissue type plasminogen activator (t-PA) after an i. v. bolus injection was studied in rabbits. t-PA with a low content of mannose (mant-PA) was obtained by treatment with α-mannosidase, which cleaves terminal mannose from the carbohydrate side chains of the molecule. About 50% of the total mannose was removed from the native t-PA.

Clearance of t-PA in rabbits was followed by measurements of both the fibrinolytic activity of the activators and the radioactivity of the iodine-labelled compounds. A biphasic mode of elimination of the fibrinolytic activity as well as of the radioactivity was found with both the native and the carbohydrate modified t-PA. The initial half-life (t_½α) was the same, about 1.5 minutes, for both compounds, irrespective of method of analysis. The late half-life (t_½β) was also the same, about 15 minutes, for both compounds. However, the β-elimination of native t-PA could only be determined accurately from radioactivity data in the dosages tested, i.e. up to 2 mg of t-PA. No dose dependency in the elimination pattern was observed.

In gel filtration experiments, with plasma from the rabbits, it was demonstrated that in addition to fibrinolytically active t-PA and man-t-PA, both fibrinolytically inactive high molecular weight complexes and low molecular weight degradation products of the activators were present in plasma after injection of the compounds.

The second phase of elimination .(β) was much more pronounced after mannosidase treatment of the t-PA. The proportion of molecules eliminated with a longer half-life was increased 6-7 times, from about 4% for native t-PA to about 25% for carbohydrate-modified t-PA.

Thus, a simple reduction of terminal mannose in the carbohydrate side chains of t-PA does not result in altered half-lives. However, a significantly greater proportion of molecules were eliminated in the late β-phase after the carbohydrate modification. The data also support our recent finding that the mannose receptor on the liver cells is at least in part responsible for the clearance of t-PA.

Keywords

Tissue plasminogen activator - Clearance - Carbohydrate - Mannose - Rabbit

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