title: Timolol activates the enzyme activities of human carbonic anhydrase I and II.
abstract: Timolol, a beta-blocker, has been shown to be an effective ocular hypotensive agent when used alone or with carbonic anhydrase inhibitor on ocular hypertensive or open angle glaucoma patients. The effect of timolol hemihydrate on the CO(2) hydration activities of human carbonic anhydrase (HCA) I and II and their reaction mechanisms were investigated. Timolol activates the enzyme activities of HCA I and HCA II. In HCA I and II, the enzyme kinetic results clearly showed that timolol increases the value of V(max) but does not influence the value of K(m). The enzyme kinetic method showed that timolol noncompetitively activates HCA I and II activities through the formation of a ternary complex consisting of the enzyme, the substrate, and timolol. These results indicate that timolol binds apart from the narrow cavity of the active site. AutoDocking results showed that timolol binds at the entrance of the active site cavity in a region where the proton shuttle residue, His 64, of HCA I or II, is placed. The enzyme kinetic and AutoDocking results showed that timolol might weakly bind near the proton shuttle residue, His 64, to accelerate the proton transfer rate from His 64 to the buffer components. It is known that efficient activators of carbonic anhydrase possess a bulky aromatic/heterocyclic moiety and a primary/secondary amino group in their molecular structure. Timolol has a heterocyclic moiety and a secondary amino group, which are typical structures in efficient activators of carbonic anhydrase.
authors.affiliation: Department of Pharmaceutical Services, Hiroshima University Hospital, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan.
authors.forename: Ayako
authors.initials: A
authors.lastname: Sugimoto
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Ikeda
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Tsukamoto
authors.forename: Kenji
authors.initials: K
authors.lastname: Kihira
authors.forename: Manabu
authors.initials: M
authors.lastname: Ishioka
authors.forename: Junzo
authors.initials: J
authors.lastname: Hirose
authors.forename: Toshiyuki
authors.initials: T
authors.lastname: Hata
authors.forename: Haruto
authors.initials: H
authors.lastname: Fujioka
authors.forename: Yukio
authors.initials: Y
authors.lastname: Ono
mesh_terms: D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D024401:Carbonic Anhydrase I / Q000378:metabolism*; D024402:Carbonic Anhydrase II / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D006801:Humans; D013999:Timolol / Q000494:pharmacology*
chemical_list: D013999:Timolol; D024401:Carbonic Anhydrase I; D024402:Carbonic Anhydrase II