What role for membranes in determining the higher sodium pump molecular activity of mammals compared to ectotherms?

The major body organs of mammals have sodium pumps that turn over energy (A TP) three to four times faster than those of ectotherms, at the same temper ature. To examine if membranes play a role in these differences in molecula r activity, membrane cross-over experiments were performed using two repres entative species, Rattus norvegicus and Bufo marinus. Microsomal membrane o f kidney and brain displayed characteristic molecular activity differences (three- to four-fold) between the species. These molecular activity differe nces could be removed by delipidation. Pre-existing molecular activities an d differences could be restored when reconstituted with original membrane. Using the same reconstitution method, species membrane cross-over experimen ts resulted in toad sodium pumps in rat membrane significantly increasing ( approximate to 30-40%) and rat sodium pumps in toad membrane significantly decreasing (approximate to 40%) activities in both kidney and brain. Analys is of membrane composition showed reduced cholesterol content and differenc es in the fatty acids of phospholipids with higher overall unsaturation in the mammal. The scope for membranes to determine protein performance and it s broader implications for metabolism are discussed.