Mg2+-mediated change in lipid fluidity enhances the reconstituted H+-ATPase activity

The results by using ANS fluorescent probe and spin labels 5-NS show that the fluidity of L. (H+-ATPase) +Mg2+ (H+-ATPase from pig heart mitochondria reconstituted in the presence of Mg2+) is less than that of L. (H+-ATPase) -Mg2+ (proteoliposome reconstituted in the absence of Mg2+). But no significant difference in fluidity has been observed when both reconstituted systems were monitored by using spin labels 12-NS and 16-NS. This indicates that Mg2+ may cause changes in fluidity of the lipid molecules near the surfaces of the bilayers, but does not affect significantly the fluidity of the deeper layer of the reconstituted system. It is tentatively supposed that in the presence of Mg2+, enhancement of activities of reconstituted H+-ATPase may be due to the Mg2+-mediated change in physical state of the lipids in the more superficial region of lipid bilayers so as to ensure a suitable conformation of ATPase complex, thereby possessing higher activity.