Myosin P-light chain isoenzymes in the human heart: evidence for diphosphorylation of the atrial P-LC form.

We studied myosin light chains (LC) of human atrium and ventricle of normal and diseased individuals by a high-resolution 2-dimensional polyacrylamide gel electrophoresis (2D-PAGE) technique. Atrial LCs (ALC-1, ALC-2 (= P-LC)) revealed both higher molecular weights and lower isoelectric points (IEP) than their ventricular counterparts (VLC-1, VLC-2 (= P-LC)). Different P-LC forms with their distinct myosin isoenzymes have been designated as P-LC-polymorphism and myosin P-LC isoenzymes, respectively. In the dephosphorylated state two VLC-2 forms (VLC-2 and VLC-2*) with the same MW and different IEP, but only one ALC-2 form, were found. In the partially phosphorylated state ALC-2 appeared to be single- and double-phosphorylated (three spots in the 2D-PAGE), whereas the two VLC-2 forms appeared to be single-phosphorylated each (four spots in the 2D-PAGE). Phosphoryl-transfer from ATP to the P-LC forms was studied using skinned fibers incubated with MLCK (myosin light chain kinase) and (gamma-32P)ATP. Ventricular myosin P-LC isoenzyme pattern was usually the same in normal and diseased patients: the VLC-2 to VLC-2* ratio was approx. 70/30, but in one patient with valvular heart disease (VHD) the relation was 55/45 (shift to the VLC-2* form). In hypertrophied atria of VHD patients a shift of the myosin P-LC isoenzyme pattern to the VLC-2* form occurred, too.