Attempt to solubilize Na+/K+-exchanging ATPase with amphiphilic peptide PD1.

The effect of the 24-amino-acid-long peptide, PD(1), on rat cerebral cortical Na+/K+ -exchanging ATPase (EC 3.6.1.37) has been studied. Incubation of the enzyme preparation (25 degrees C for 10-25 min) with the peptide (10(-7) - 10(-4) M) did not appreciably affect the activity of the enzyme, only 5-8% activation being registered. On the other hand, PD(1) completely eliminated the cooperative nature of Na+ -binding to Na+/K+ -exchanging ATPase (n(H) decreased from 1.4 to 0.9) and slightly (1.2-fold) decreased the affinity for Na+. ATP, a substrate of activity for Na+/K+ -exchanging ATPase, blocked the PD(1)-promoted effect on the cooperativity for Na+. Incubation of cerebral cortical membranes with 5 x 10(-4) M PD(1) revealed a shift (from 19.5 degrees C to 21.4 degrees C) of the typical break on the Arrhenius plot (15-37 degrees C). Prolonged incubation of enzyme preparation (25 degrees C for 1-2 h) with PD(1) (4.5 x 10(-4) - 0.7 x 10(-2) M) followed by centrifugation of the mixture at 53,000 g for 90 min, resulted in loss of the activity both in the supernatant and the sediment, while the protein content in the supernatant and the sediment remained unchanged. After a short incubation (25 degrees for 10 min) with PD(1) (1 x 10(-6) M), followed by centrifugation, the full activity of Na+/K+ -exchanging ATPase in the sediment was restored. These data suggest that peptitergent PD(1) does not solubilize the transmembrane protein Na+/K+ -exchanging ATPase, although it abolishes the cooperative effect of Na+.