Localization and Kinetic Properties of {beta}-Glycerophosphatase in Barley Roots

The study of ß-glycerophosphatase activity in cell-wallpreparations and in excised root tips from barley seedlingssupports the view that the former, which constitutes about 20per cent of the activity of the whole homogenate, representsthe fraction located at the surface of the roots in vivo. Theactivities of the cell-wall suspension and intact roots arevirtually identical, and further show identical relations topH, substrate concentration (K_m), and competitive inhibitionby molybdate and inorganic phosphate (K_i). The enzyme must thereforebe freely exposed to the external solution without any permeabilitybarrier separating it from either substrate or inhibitors. Theabsence of any lag phase in the hydrolysis in excised root tipssuggests that the surface enzyme may be limited to the outermostlayers of the root. The solubilization of some of the activityof the cell-wall preparation by treatment with sodium chlorideand ammonium sulphate suggest that surface activity may havebeen lost from these preparations rather than adsorbed duringhomogenization and extraction. The K_m and pH-activity curveof the supernatant activity remaining after centrifugation ofthe cell-wall fraction indicate that only this enzyme and noother detectable glycerophosphatase exists in the roots.