[SH-group and Mg2+-dependent Ca2+-activated ATPase activity of the microsome fraction of the brain]

Mg2+-dependent Ca2+-activated ATPase of microsoma fraction from the grey matter of cerebral great hemispheres determined after the preliminary treatment of the preparation with 0.1% digitonin, while preserved in the medium with 10 mM mercaptoethanol for seven days at a temperature of 4-6 degrees C is inactivated by 10-15% and approximately by 50% while preserved without mercaptoethanol. Mercaptoethanol does not make reactivating effect. SH-reagents at definite concentrations completely inhibit the activity of Mg2+, Ca2+-ATPase. Half-maximum inhibition of the enzyme is reached with the salirgan, p-CMB and NEM concentrations of 5-10(-6) M, 5-10(-6) M and 5-10(-3) M, respectively. Mg2+-ATPase is not suppressed completely, and at high concentrations of SH-reagents the residual activity is 1.3 muM of Pi per 1 mg of protein in 1 hr. ATP in the concentrations optimal for manifestation of Mg2+, Ca2+-ATPase (3 mM) efficiently protects the enzyme from the inactivating effect of NEM. This gives reasons to suppose that the active centre of Mg2+, Ca2+-ATPase contains an SH-group. The quantity of SH-groups readily accessible of the Ellman reactive in the initial preparation of the brain microsomes is 45 + 2.0 nM per 1 mg of protein and in the preparation dissolved in 2.5% sodium dodecyl sulphate, 110 + 7.8 nmM per 1 mg of protein. In the presence of 0.1% digitonin the quantity of SH-groups of the preparation is 55 + 3.5 nM per 1 mg of protein, simultaneously such treatment of detergent results in manifestation of Mg2+, Ca2+-ATPase activity. An inactivating effect of SH-reagents and the protective effect of ATP indicate similarity of the enzyme under study to Mg2+, Ca2+-ATPase of sarcoplasmatic reticulum.