title: Evidence for histidyl and carboxy groups at the active site of the human placental Na+-H+ exchanger.
abstract: The Na+-H+ exchanger of the human placental brush-border membrane was inhibited by pretreatment of the membrane vesicles with a histidyl-group-specific reagent, diethyl pyrocarbonate and with a carboxy-group-specific reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline. In both cases the inhibition was irreversible and non-competitive in nature. But, if the membrane vesicles were treated with these reagents in the presence of amiloride, cimetidine or clonidine, there was no inhibition. Since amiloride, cimetidine and clonidine all interact with the active site of the exchanger in a mutually exclusive manner, the findings provide evidence for the presence of essential histidyl and carboxy groups at or near the active site of the human placental Na+-H+ exchanger. This conclusion was further substantiated by the findings that Rose Bengal-catalysed photo-oxidation of histidine residues as well as covalent modification of carboxy residues with NN'-dicyclohexylcarbodi-imide irreversibly inhibited the Na+-H+ exchanger and that amiloride protected the exchanger from inhibition caused by NN'-dicyclohexylcarbodi-imide.
authors.affiliation: Department of Cell and Molecular Biology, Medical College of Georgia, Augusta 30912.
authors.forename: V
authors.initials: V
authors.lastname: Ganapathy
authors.forename: D F
authors.initials: DF
authors.lastname: Balkovetz
authors.forename: M E
authors.initials: ME
authors.lastname: Ganapathy
authors.forename: V B
authors.initials: VB
authors.lastname: Mahesh
authors.forename: L D
authors.initials: LD
authors.lastname: Devoe
authors.forename: F H
authors.initials: FH
authors.lastname: Leibach
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D001665:Binding Sites; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004024:Dicyclohexylcarbodiimide / Q000494:pharmacology; D004047:Diethyl Pyrocarbonate / Q000494:pharmacology; D005260:Female; D006801:Humans; D007700:Kinetics; D008871:Microvilli / Q000378:metabolism; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011804:Quinolines / Q000494:pharmacology; D012395:Rose Bengal / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers
chemical_list: D002352:Carrier Proteins; D011804:Quinolines; D017923:Sodium-Hydrogen Exchangers; D012395:Rose Bengal; D004024:Dicyclohexylcarbodiimide; C000470:EEDQ; D000584:Amiloride; D012964:Sodium; D004047:Diethyl Pyrocarbonate