title: Mammalian forebrain ketimine reductase identified as μ-crystallin; potential regulation by thyroid hormones.
abstract: Ketimine reductase (E.C. 1.5.1.25) was purified to apparent homogeneity from lamb forebrain by means of a rapid multi-step chromatography protocol. The purified enzyme was identified by MS/MS (mass spectrometry) as μ-crystallin. The identity was confirmed by heterologously expressing human μ-crystallin in Escherichia coli and subsequent chromatographic purification of the protein. The purified human μ-crystallin was confirmed to have ketimine reductase activity with a maximum specific activity similar to that of native ovine ketimine reductase, and was found to catalyse a sequential reaction. The enzyme substrates are putative neuromodulator/transmitters. The thyroid hormone 3,5,3'-l-triiodothyronine (T3) was found to be a strong reversible competitive inhibitor, and may have a novel role in regulating their concentrations. μ-Crystallin is also involved in intracellular T3 storage and transport. This research is the first to demonstrate an enzyme function for μ-crystallin. This newly demonstrated enzymatic activity identifies a new role for thyroid hormones in regulating mammalian amino acid metabolism, and a possible reciprocal role of enzyme activity regulating bioavailability of intracellular T3.
authors.affiliation: Department of Chemistry and Biomolecular Sciences, Macquarie University, North Ryde, New South Wales, Australia.
authors.forename: André
authors.initials: A
authors.lastname: Hallen
authors.forename: Arthur J L
authors.initials: AJ
authors.lastname: Cooper
authors.forename: Joanne F
authors.initials: JF
authors.lastname: Jamie
authors.forename: Paul A
authors.initials: PA
authors.lastname: Haynes
authors.forename: Robert D
authors.initials: RD
authors.lastname: Willows
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000201:enzymology; D001923:Brain Chemistry / Q000502:physiology; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D003459:Crystallins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D004591:Electrophoresis, Polyacrylamide Gel; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D000587:Oxidoreductases Acting on CH-NH Group Donors / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D016548:Prosencephalon / Q000201:enzymology*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000235:genetics; D012756:Sheep; D013056:Spectrophotometry, Ultraviolet; D013963:Thyroid Hormones / Q000502:physiology*; D014284:Triiodothyronine / Q000378:metabolism; D000090803:mu-Crystallins
chemical_list: D003459:Crystallins; D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D013963:Thyroid Hormones; D014284:Triiodothyronine; D000587:Oxidoreductases Acting on CH-NH Group Donors; C056073:ketimine reductase; D000090803:mu-Crystallins