Effect of K super(+), and other ligands on the thiol reactivity and tryptic cleavage pattern of scallop sarcoplasmic reticulum

The kinetics of reaction of the thiol groups of both membranous and non-ionic detergent-solubilized Ca-ATPase of scallop sarcoplasmic reticulum towards 5,5'-dithiobis(2-nitrobenzoate) were greatly in different ways by the presence of the following combinations of ligands: Ca super(2+), EGTA (no Ca super(2+)), (ATPMg super(2-) + EGTA) and (ATP + Ca super(2+)). K super(+) was found to influence greatly the pattern of reactivity of the thiol groups of the scallop Ca-ATPase, modifying the kinetics of reaction differently according to the types of other ligand present. While all the thiol groups on the non-ionic detergent-solubilized Ca-ATPase were available for reaction in the absence of K super(+), whatever the combination of ligands, in the presence of K super(+), several groups became completely unreactive towards the reagent. Large differences were also seen in the tryptic cleavage pattern in the presence of the different ligands, and K super(+) led to major modifications in the products of digestion in the absence of nucleotide. A complete amino-acid analysis of the scallop Ca-ATPase was carried out.