title: pH-sensitive control of arginase by Mn(II) ions at submicromolar concentrations.
abstract: The manganese dependence of arginase was reinvestigated with extracts of mouse liver to see whether more physiological properties were displayed than have been reported for the purified enzyme. In a preincubation with Mn(II) ions at 37 degrees C the enzyme underwent a slow and reversible activation. At least 90-95% of the activation achieved was dependent on Mn2+. However, no Mn2+ was required for catalytic activity in the assay. The activation showed little dependence upon pH over the range 6.5-9.5, whereas the catalytic activity increased 12-fold in apparent accord with the titration curve of an ionizable group of pKa 7.9. The Mn2+ dependence of arginase activation obeyed Michaelis-Menten kinetics, with Kd varying from 0.3 microM at pH 6.8 to 0.08 microns at pH 7.7. Free Mn2+ concentrations were established in these assays with a trimethylenediaminetetraacetate-Mn buffer. Vmax increased about three-fold over this range. The calculated arginase activity at 0.05 microM Mn2+ increases about nine-fold over this physiological pH range. An enzyme model is proposed to explain these findings. The activity of arginase at "physiological" [Mn2+] and the pronounced pH dependence conferred upon it are consistent with a recently revised role for the urea cycle in the control of bicarbonate and pH in the body. It appears possible that arginase loses Mn2+ sensitivity during the usual purification.
authors.affiliation: School of Biochemistry, University of Birmingham, United Kingdom.
authors.forename: N J
authors.initials: NJ
authors.lastname: Kuhn
authors.forename: J
authors.initials: J
authors.lastname: Talbot
authors.forename: S
authors.initials: S
authors.lastname: Ward
mesh_terms: D000818:Animals; D001119:Arginase / Q000302:isolation & purification / Q000378:metabolism*; D002712:Chlorides; D004492:Edetic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008345:Manganese / Q000494:pharmacology*; D017895:Manganese Compounds; D051379:Mice; D008954:Models, Biological
chemical_list: D002712:Chlorides; D017895:Manganese Compounds; D008345:Manganese; D004492:Edetic Acid; D001119:Arginase; C025340:manganese chloride