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-a lkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), a ke y enzyme in the biosynthesis of platelet-activating factor (PAF, 1-alk yl-2-acetyl-sn-glycero-3-phosphocholine). The assay involves measureme nt of the radioactivity in the trichloroacetic acid (TCA)-precipitated complex of radioactive product and albumin after incubation of 1-alky l-sn-glycero-3-phosphocholine and [H-3]acetyl-CoA with rat spleen micr osomes or membrane fractions of human polymorphonuclear leukocytes (PM Ns). The radioactive product associated with the precipitate was ident ified as PAF using an ultrahigh-sensitivity TV camera system after ext raction 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, i nhibited the acetyltransferase activity in rat spleen microsomes (IC50 , 150 and 150 muM, respectively) and membrane fractions of human PMNs (IC50, 70 and 60 muM, respectively). The inhibitory action of magnolol and honokiol was reversible, and similar to or higher than that of no rdihydroguaiaretic acid. PAF production in human PMNs stimulated by th e ionophore A23187 was also suppressed dose dependently by magnolol an d honokiol. These activities may be relevant to the claimed therapeuti c effects of the extract from Magnoliae cortex.