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Abstract

In the reaction of sarcoplasmic reticulum membranes with excess 5,5'-dithiobis(2-nitrobenzoate) (DTNB) some new features were observed: The Ca2+-dependent ATPase activities of increasingly modified preparations were considerably enhanced during the initial stage of thiol blockage. A maximum (130-160% of the control activity) was reached when about 1.5-2 mol thiol groups per 10(5) g vesicular protein had reacted, in the absence of ATP and detergent. At higher extents of modification inactivation occurred. Purified ATPase behaved principally similar to native sarcoplasmic vesicles. In the presence of Mg2+ and ATP the activity maximum (up to 180% of control) was broadened and shifted towards a higher degree of thiol blockage. Concomitantly the modification and inactivation rates were considerably reduced. Glycerol (10-30%, v/v) slightly enhanced the ATPase activity maximum and reduced the rate of inactivation essentially only by decreasing the DTNB modification rate. In the presence of sufficient myristoylglycerophosphocholine for solubilization no activation was observed. The steady state level of phosphoprotein from ATP was raised to about 150% of the control level 10 s after addition of DTNB (about 1/2 thiol blocked), followed by a linear decrease with the number of thiols labeled, while the Ca2+-dependent ATPase activity of preparations modified under equivalent conditions (10(-4) M Ca2+ and 2 X 10(-3) M Mg2+ present) showed a broader maximum corresponding to 1.5 thiols blocked.