ATP-dependent calcium sequestration and calcium/ATP stoichiometry in isolated microsomes from guinea pig parotid glands

AbstractATP-dependent calcium uptake was studied in isolated guinea pig parotid gland microsomes. The apparent Km for free Ca2+ was 0.41 μM, the apparent Km for ATP·Mg2− 0.23 mM. The pH optimum was 6.8–7.0. Subfractionation of the microsomes revealed that the highest specific uptake activity resided in a rather dense fraction of the endoplasmic reticulum. The calcium uptake/ATPase stoichiometry was determined in the absence of exogenous magnesium in the submicrosomal fractions. It ranged from 1–2. It is concluded that in vivo the stoichiometry is the same as in sarcoplasmic reticulum, namely 2