An Acidic Kininogenase in Rat Ventricular Myocardium

Abstract

Background: Canine ventricles have been reported to contain a cathepsin D-like kininogenase, which might confer protection on the heart during ischaemia. The aim of this study was to investigate the presence and levels of a similar kininogenase in normal and ischaemic rat hearts.

Methods: Aqueous extracts of rat ventricles were tested for the ability to release bradykinin-like immunoreactivity from human low-molecular-weight kininogen and high-molecular-weight kininogen at acidic pHs. The enzymes involved were separated using gel filtration followed by the testing of fractions for cleavage of D-Val-Leu-Arg-pNA and low-molecular-weight kininogen. Extracts from normal and ischaemic ventricles were compared for the ability to release bradykinin-like immunoreactivity from low-molecular-weight kininogen. Kinin levels in mixed venous blood were compared before and after ischaemia. By assessing their effect on isolated oestrous rat uteri and on protease inhibitors, further characterization of acidic kininogenases in the extracts was performed.

Results: Extracts of rat ventricles released bradykinin-like immunoreactivity only from low-molecular-weight kininogen. Using the isolated oestrous rat uterus, gel filtration and protease inhibitors, the enzyme involved was identified as a cathepsin D-like enzyme with an optimum pH of 4.7 and a molecular weight of 42.8 ± 4.9 kDa. It is an arginine amidase and releases bradykinin-like immunoreactivity from low-molecular-weight kininogen. Ischaemia reduced the amount of bradykinin-like immunoreactivity released by the ventricular extract (P ≤ 0.05) and increased levels of free kinin in venous blood from the right atrium.

Conclusion: Rat ventricles contain a cathepsin D-like acidic protease that cleaves low-molecular-weight kininogen to release bradykinin-like immunoreactivity. The acidic protease may protect the heart during ischaemia.