The influence of phosphatidate bilayers on pig heart AMP deaminase. Crucial role of pH-dependent lipid-phase transition

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Abstract
Phosphatidate bilayers composed of dilauroylphosphatidate, dimyristoylphosphatidate, dipalmitoylphosphatidate and dioleoylphosphatidate were prepared. Their interaction with AMP deaminase isolated from pig heart was investigated. Dioleoylphosphatidate bilayers were found to exert non-competitive inhibition on the AMP deaminase with a Ki of 15 .times. 10-6 M. This inhibition is three orders of magnitude stronger than that exerted by orthophosphate. The phosphatidate species containing saturated fatty acids were either non-inhibitory or inhibited enzyme activity rather poorly. However, alkalinization of the medium from pH 6.5 to pH 7.9 led to the inhibition of pig heart AMP deaminase by dilauroylphosphatidate bilayers. This was accompanied by the fluidization of the saturated phosphatidate species, i.e. the lowering of their phase transition temperature in alkaline pH, as measured by light-scattering and fluorescence scans. The possible significance of these findings for the regulation of AMP deaminase activity in vivo by natural membranes is discussed.