Reactivity of sarcoplasmic reticulum from rabbit cardiac muscle with 1-fluoro- and 1,5-difluoro- 2,4-dinitrobenzene.

Sarcoplasmic reticulum preparations from rabbit cardiac and fast skeletal muscle react differentially with low concentrations of 1-fluoro- and 1,5-difluoro-2,4-dinitrobenzene. Dinitrophenylation of cardiac sarcoplasmic reticulum by 1-fluoro-2,4-dinitrobenzene is not affected by Ca2+ and is limited to the lipoprotein-lipid region. This contrasts sharply with the predominant Ca2+-dependent dinitrophenylation of the ATPase protein of rabbit skeletal sarcoplasmic reticulum by this reagent. Formation of non-serial high mol. wt. oligomers by 1,5-difluoro-2,4-dinitrobenzene is significantly greater in cardiac than in skeletal vesicles. Substrate MgATP2- does not protect rabbit cardiac sarcoplasmic reticulum ATPase activity or Ca2+ uptake from dinitrophenylation when monofunctional and bifunctional reagents are used. Chemical differences in the overall structure of the two kinds of membrane preparations can be ascertained from a comparison of the effects of Ca2+ and MgATP2- on the reactivity of these reagents.