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Medical Definition of Apolipoproteins a
1. Lipoproteins found in human blood serum in the high-density and very-high-density lipoprotein fraction (hdl, vhdl). They consist of several different polypeptides, the most important of which are apolipoprotein a-I and a-II. They maintain the structural integrity of the hdl particles and are activators of lecithin:cholesterol acyltransferase (lcat). Atherosclerotic patients show low apolipoprotein a levels and these apolipoproteins are either absent or present in extremely low plasma concentration in tangier disease. (12 Dec 1998)