Evidence for essential disulfide bonds in the beta-subunit of (Na+ + K+)-ATPase

(Na+ + K+)-ATPase from dog kidney lost its activity when heated at 55 degrees C in the presence of 0.3 M 2-mercaptoethanol. Either heat treatment alone or addition of reducing agent at around 25 degrees C caused little inactivation. One disulfide bond per protomer (mol. wt. 146 000) was reduced in the inactivated sample but in active samples no reduction occurred. Neither K+-dependent phosphatase activity nor phosphoenzyme formation in the presence of Na+ was detected in the inactivated sample, suggesting that the disulfide bond was essential for the catalytic cycle of (Na+ + K+)-ATPase. This essential disulfide bond belonged to the beta-subunit, the glycoprotein component of the enzyme, indicating that the beta-subunit may be an integral component of the (Na+ + K+)-ATPase system.