BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS HETEROGENEITY OF St1 GROUPS IN SARCOPLASMIC RETICULUN

14 S W Y : The incorporation of [ C ] N-ethylmaleimide reveals fast and slowreacting sulfhydryl groups in sarcoplasmic reticulum. Two proteins react with the label: a fast-reacting glycoprotein recently isolate$+(Ikemoto, Cucchiaro and Garcia (1976) J. Cell Biol. 10, 290a), and the Ca -ATPase. Labeling sarcoplasmic reticulum with a maleimide spin label gives a similar pattern. The spectra of maleimide-spin-labeled sarcoplasmic reticulum have both 'strongJyl and 'weakly' inmobilized components. Maleimide-spin-labeled purified Ca ATPase, ot sarcoplasmic reticulum labeled first with N-ethylmaleimide, and then with maleimide spin label, show spectra devoid of the 'weakly' inrmobilized component; the latter is enhanced in partially purified glycoprotein obtained from spin-labeled sarcoplasmic reticulum. This indicates that spectra from maleimide-spinabeled sarcoplasmic reticulum do not reflect exclusively the J+ state of the Ca, -ATPase enzyme. 1 . Covalent labeling of the sulfhydryl groups present in fragmented SR wlth