Isolation and characterization of myosin heavy chain isozymes of the bovine conduction system

Abstract

To determine the characteristics of cardiac myosin in the conduction system, a pure Purkinje fiber preparation, consisting of atrioventricular nodes and the ventricular conduction system, was obtained from bovine hearts. Two types of myosin heavy chain isozymes, α-type and β-type, were fractionated by affinity chromotography using monoclonal antibodies CMA19 and HMC50, which are specific for the α-type heavy chain and β-type heavy chain, respectively. Competitive enzyme-linked immunosorbent assay demonstrated that the content of β-type in the atrioventricular node (30-40%) was higher than that in atrial ordinary myocardium (10-20%) and that of the α-type was 30-40% in the ventricular conduction system, which was much higher than that in the ventricular ordinary myocardium (less than 10%). By one- and two-dimensional electrophoresis of the peptides produced by partial and complete digestion, the peptide compositions of α-type and β-type in the conduction system were shown to be very similar to those of α-type and β-type in ordinary myocardium, respectively. The Ca2+-activated ATPase activity of myosin of the atrioventricular nodes was lower than that of ordinary atrial myosin (0.46 ± 0.03 versus 0.58 ± 0.02 μmol Pi/mg/min, mean ± SEM, p < 0.05) and in contrast, that of ventricular specialized myocardium was higher than that of myosin in the ventricular ordinary working myocardium (0.32 ± 0.03 versus 0.22 ± 0.01 μmol Pi/mg/min, p < 0.05). This was in good agreement with the relative proportion of myosin isozymes. We concluded that there were two distinctive myosin heavy chain isozymes in the conduction system and that they were closely related to heavy chain α and β of the ordinary working myocardium.