When incubated with intact erythrocytes, low density lipoproteins (LDL) decrease the phosphate content of erythrocyte spectrin allowing the cells to undergo morphological transformation. The phosphate content of spectrin depends on the balance between the activity of membrane-associated cyclic AMP-independent protein kinases and phosphoprotein phosphates. LDL do not influence the activity of membrane-associated cyclic AMP-independent protein kinases; these lipoproteins activate by 2-fold and greater membrane-associated phosphatases as determined by hydrolysis of p-nitrophenyl phosphate and by phosphate hydrolysis of phosphorylated erythrocyte membrane proteins. We conclude that LDL interact at the exterior surface of the erythrocyte to stimulate dephosphorylation of spectrin. The significance of this conclusion is augmented by the fact that spectrin, the target for LDL-induced dephosphorylation, specifies cell morphology and modulates the distribution of cell-surface receptors. LDL also render erythrocyte acetylcholinesterase less susceptible to inhibition by F-. Lipoproteins in the high density class (HDL) do not stimulate dephosphorylation of spectrin, and they are consequently unable to alter erythrocyte morphology. HDL do prevent the LDL-induced activation of membrane phosphatase. The inhibitory capacity of HDL is observed over the range of LDL : HDL (w/w) which exists in the plasma of normolipemic humans. © 1979.
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