Substrate specificity of bacterial glycerophospholipid:cholesterol acyltransferase

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Abstract
The substrate specificity of a bacterial [Aeromonas salmonicida] analog of the [mammalian] plasma enzyme lecithin:cholesterol acyltransferase (LCAT) was examined with small unilamellar liposomes and triton mixed micelles. In contrast to LCAT, the microbial enzyme is capable of using all of the naturally occurring phospholipids as acyl donors. In general reaction rate depends more on the length or degree of unsaturation of the acyl chains than on the nature of the phospholipid head group. Among a series of disaturated phosphatidylcholines in liposomes, dilauroylphosphatidylcholine is the preferred acyl donor. Like LCAT, the enzyme will catalyze acyl transfer by using other alcohols in addition to cholesterol. Of saturated straight chain primary alcohols 1-decanol is the preferred acyl acceptor. Cholesterol, however, is a far better acceptor than any nonsterol alcohol tested. Other steroids with equatorial hydroxyls at position C-3 and trans-fused A:B rings will also act as acceptors whereas those steroids with axial hydroxyls at C-3 or cis-fused rings are inhibitors of acyl transfer. The ability of steroids to act as acyl acceptors may be due to the nature of their interaction with the phospholipid acyl donor.