Monoclonal antibodies to dog heart sarcoplasmic reticulum. Antibodies that inhibit Ca2+-pump systems of cardiac and skeletal muscles.

Purified sarcoplasmic reticulum (SR) vesicles from dog heart were used as an antigen to produce monclonal antibodies (mAbs) to the Ca2+-ATPase. Nine of twelve clones of hybridoma cells produce mAbs which cross-react with seven SR preparation isolated from cardiac and skeletal muscles of various species. Three mAbs of IgM type interact with the 45-kDa tryptic fragment of rabbit skeletal muscle Ca2+-ATPase and markedly inhibit Ca2+ uptake (by 95%) and ATPase activity (by 80%) and decrease (by 30–50%) the steady-state level of the Ca2+-ATPase phosphoenzyme. The ATPase activity could be completely blocked by one of these mAbs if the incubation medium was supplemented with 2 μM orthovanadate. On the other hand, when SR vesicles were treated with increasing concentrations of a nonioinic detergent C12E8, the inhibiting effect of mAb 4B4 is diminished. It is concluded that the mAbs inhibit the Ca2+-ATPase only if the enzyme exists in an oligomeric form. The inhibition of the SR activities is due to an effect of the mAbs on the whole active center of the enzyme, rather than on a single partial reaction.