Oxidosqualene cyclase involved in the biosynthesis of alpha-onocerin, a triterpenoid characterized by a symmetrical structure due to cyclizations at both termini of dioxidosqualene that inhibits acetylcholinesterase. Catalyzes the second half of the cyclization, exclusively from pre-alpha-onocerin. pre-alpha-onocerin = alpha-onocerin Secondary metabolite biosynthesis; terpenoid biosynthesis. Belongs to the terpene cyclase/mutase family.