LIPIDS ASSOCIATED WITH THE (NA-K+)ATPASE ACTIVITY OF NORMAL AND DENERVATED ELECTRIC ORGANS OF ELECTROPHORUS-ELECTRICUS (L)()

The effect of denervation on the lipid metabolism and on the activity of (Na+-K+)ATPase isoforms from the membrane fraction P-3, which corre sponds to the innervated electrocyte membrane, was evaluated. On a dis continuous sucrose gradient, normal P-3 membranes exhibit a bimodal (' 'a'' and ''b'' bands) distribution of the (Na+-K+)ATPase activity, whi ch upon denervation changes to an unimodal (''c'' band) distribution. Using these fractions, which have a higher (Na+-K+)ATPase activity, we characterized the lipids at the hydrophobic protein surface boundary, (i.e., the bulk lipids that surround the protein). The results confir m that these lipids consist of phospholipids and cholesterol. The quan titative composition of the phospholipids is similar for both isoform fractions obtained from the discontinuous gradient of normal membranes , with phosphatidylcholine, phosphatidylethanolamine and phosphatidyls erine representing about 90% of the total phospholipids. Sphingomyelin , phosphatidylinositol, diphosphatidylglycerol and phosphatidic acid w ere in the minority. However, in the single band obtained after denerv ation, the three major phospholipid components decreased to 70% of the total, and a significant increase in the other phospholipids and in c holesterol was observed. The high cholesterol content of the denervate d fraction may confer membrane stabilization, as it is likely to cause a decrease in the membrane fluidity and consequently in the enzyme ac tivity.