Development of a novel method for determination of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase activity and its application to screening for acetyltransferase inhibitors. Inhibition by magnolol and honokiol from Magnoliae cortex.

A method was developed for determining the activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), a key enzyme in the biosynthesis of platelet-activating factor (PAF, 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine). The assay involves measurement of the radioactivity in the trichloroacetic acid (TCA)-precipitated complex of radioactive product and albumin after incubation of 1-alkyl-sn-glycero-3-phosphocholine and [3H]acetyl-CoA with rat spleen microsomes or membrane fractions of human polymorphonuclear leukocytes (PMNs). The radioactive product associated with the precipitate was identified as PAF using an ultrahigh-sensitivity TV camera system after extraction and separation by TLC. This TCA method was then used to screen the components of crude preparations that inhibited acetyltransferase activity. Major components from the cortex of Magnoliae (magnolol and honokiol), which have anti-inflammatory and anti-bacterial actions, inhibited the acetyltransferase activity in rat spleen microsomes (IC50, 150 and 150 microM, respectively) and membrane fractions of human PMNs (IC50, 70 and 60 microM, respectively). The inhibitory action of magnolol and honokiol was reversible, and similar to or higher than that of nordihydroguaiaretic acid. PAF production in human PMNs stimulated by the ionophore A23187 was also suppressed dose dependently by magnolol and honokiol. These activities may be relevant to the claimed therapeutic effects of the extract from Magnoliae cortex.