CHOLESTEROL OXIDATION REDUCES CA2-ATPASE ACTIVITY, INTERDIGITATION, AND INCREASES FLUIDITY OF BRAIN SYNAPTIC PLASMA-MEMBRANES(+MG2+)

These experiments examined effects of cholesterol oxidation on Ca2+ Mg2+-ATPase activity, Na+ + K+-ATPase activity, and membrane structure of brain synaptic plasma membranes (SPM). Cholesterol oxidase [E.C.1. 1.3.6 from Brevibacterium sp.] was used to oxidize cholesterol. Two ch olesterol pools were identified in synaptosomal membranes based on the ir accessibility to cholesterol oxidase. A rapidly oxidized cholestero l pool was observed with a (1)t(1/2) of 1.19 +/- 0.09 min and a second pool with a (2)t(1/2) of 38.30 +/- 4.16 min. Activity of Ca2+ + Mg2+- ATPase was inhibited by low levels of cholesterol oxidation. Ten perce nt cholesterol oxidation, for example, resulted in approximately 35% p ercent inhibition of Ca2+ + Mg2+-ATPase activity. After 13% cholestero l oxidation, further inhibition of Ca2+ + Mg2+-ATPase activity was not observed. Activity of Na+ + K+-ATPase was not affected by different l evels of cholesterol oxidation (5%-40%). SPM interdigitation was signi ficantly reduced and fluidity was significantly increased by cholester ol oxidation. The relationship observed between SPM interdigitation an d Ca2+ + Mg2+-ATPase activity was consistent with studies using model membranes [7]. Brain SPM function and structure were altered by relati vely low levels of cholesterol oxidation and is a new approach to unde rstanding cholesterol dynamics and neuronal function. The sensitivity of brain SPM to cholesterol oxidation may be important with respect to the proposed association between oxygen free radicals and certain neu rodegenerative diseases.