Affinity labeling of lecithin retinol acyltransferase

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Abstract
Lecithin retinol acyltransferase (LRAT) transfers acyl groups regiospecifically from the sn-1 position of lecithins to all-trans-retinol (vitamin A) and similar retinoids. LRAT is essential for the biosynthesis of 11-cis-retinal, the visual pigment chromophore. LRAT is also required for the general dietary mobilization of vitamin A. The enzyme is membrane-bound and has been solubilized and partially, but not completely, purified. It is demonstrated here that all-trans-retinyl alpha-bromoacetate (RBA) is a potent irreversible affinity labeling agent of LRAT. The measured KI = 12.1 microM and the pseudo-first-order rate constant for inhibition is kinh = 8.2 x 10(-4) s-1. The specificity of the inhibition process is further evidenced by the observation that alpha-bromoacetate derivatives of hydrophobic alcohols which are not substrates for LRAT, such as cholesterol and beta-ionol, are not inhibitors of the enzyme. Labeling of the partially purified enzyme with 3H-RBA showed a single radiolabeled band of molecular weight approximately 25,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.