| Research Abstract |
Brain intracellular platelet-activating factor acetylhydrolase (PAF-AH) isoform I is a member of a family of complex enzymes composed of mutually homologous α1 and α2 subunits, both of which account for catalytic activity, and the b subunit. We previously demonstrated that the expression of one catalytic subunit, α1, is developmentally regulated, resulting in a switching of the catalytic complex from α1/α2 during brain development (Manya, H., Aoki, J., Watanabe, M., Adachi, T., Asou, H., Inoue, Y., Arai, H., and Inoue, K. (1998) J. Biol. Chem. 273, 18567 - 18572). In this study, we explored the biochemical differences in three possible catalytic dimers, α1/α2, and α2/α2 homodimer exhibited different substrate specificity from the α1/α2 heterodimer, both of which showed similar substrate specificity. The α2/α2 homodimer hydrolyzed PAF and 1-O-alkyl-2-acetyl-sn-glycero-3-phosphophatidylethanolamine (AAGPE) most efficiently among 1-O-alkyl-2-acetyl-phospholipids. In contrast, both α1/α1 and α1/α2 hydrolyzed 1-O-alkyl-2-acetyl-sn-glycero-3-phosphophatidic acid (AAGPA) more efficiently than PAF. AAGPE was the poorest substrate for these enzymes. The β subunit bound to all three catalytic dimers but modulated the enzyme activity in catalytic dimer composition-dependent manners. The βsubunit strongly accelerated the enzyme activity of the α2/α2 homodimer but rather suppressed the activity of the α1/α1 homodimer and had little effect on that of the α1/α2 heterodimer. The (His 149 to Arg 149) mutant β, which has been recently identified in isolated lissencephaly sequence patients, lost the ability to either associate with the catalytic complexes or modulate their enzyme activity. The enzyme activity of PAF-AH isoform I may be regulated in multiple ways by switching the composition of the catalytic subunit and by manipulating the β subunit.
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