UREOLYTIC RUMEN BACTERIA: II. EFFECT OF INORGANIC IONS ON UREASE ACTIVITY

Abstract

The urease activity of whole cell preparations of the mixed rumen microflora was reduced when the cells were washed with maleate buffer. Resuspension of the washed cells in rumen supernatant liquor (RSL) enhanced their urease activity owing to the presence of an inorganic urease-stimulating factor in RSL. When the whole cells were washed in buffer there was a progressive reduction in the urease activity of the cells as the number of washings increased. This effect was attributed to progressive removal from the cells of the urease-stimulating factor. The urease activity of preparations of whole cells washed three times in buffer was stimulated in the presence of Mn²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, and inhibited in the presence of Na⁺, K⁺, and Co²⁺. These ions brought about similar effects on the urease activity of acetone-dried powders and cell-free extracts of washed rumen microorganisms. The activity of jack-bean urease was reduced in the presence of the ions which stimulated rumen urease.When inorganic ions were added individually to suspensions of washed rumen microorganisms there were time lags before the ions exerted their stimulating or inhibiting effects on urea hydrolysis. Once the stimulation or inhibition of activity by an ion had begun, however, the reaction proceeded at a constant rate which was characteristic of the ion tested. Modification of the permeability properties of washed cells of rumen microorganisms by acetone treatment modified but did not eliminate the time lags. When a cell-free extract of washed cells was tested, Mn²⁺, Mg²⁺, Ca²⁺, Co²⁺, and K⁺all influenced urease activity immediately upon addition to the extract, whereas Sr²⁺, Ba²⁺, and Na⁺were active only after a lag period.The urease produced by rumen bacteria is considered to be a metal-activated, intracellular enzyme. The urease-stimulating factor present in the cells was not bound to any appreciable extent intracellularly. The time lags observed with the whole cells were attributable in most cases to the time taken for the ions to penetrate the cells; with Sr²⁺, Ba²⁺, and Na⁺they were apparently due, at least in part, to a delay in the formation of an active complex between ion and enzyme.