PH-SENSITIVE CONTROL OF ARGINASE BY MN(II) IONS AT SUBMICROMOLAR CONCENTRATIONS

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 °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 pKα 7.9. The Mn2+ dependence of arginase activation obeyed Michaelis-Menten kinetics, with Kd varying from 0.3 μm at pH 6.8 to 0.08 μm 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 μm 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. © 1991."