Localization and Kinetic Properties of β-Glycerophosphatase in Barley Roots

Abstract

The study of β-glycerophosphatase activity in cell-wall preparations and in excised root tips from barley seedlings supports the view that the former, which constitutes about 20 per cent of the activity of the whole homogenate, represents the fraction located at the surface of the roots in vivo. The activities of the cell-wall suspension and intact roots are virtually identical, and further show identical relations to pH, substrate concentration (Km), and competitive inhibition by molybdate and inorganic phosphate (Ki). The enzyme must therefore be freely exposed to the external solution without any permeability barrier separating it from either substrate or inhibitors. The absence of any lag phase in the hydrolysis in excised root tips suggests that the surface enzyme may be limited to the outermost layers of the root. The solubilization of some of the activity of the cell-wall preparation by treatment with sodium chloride and ammonium sulphate suggest that surface activity may have been lost from these preparations rather than adsorbed during homogenization and extraction. The Km and pH-activity curve of the supernatant activity remaining after centrifugation of the cell-wall fraction indicate that only this enzyme and no other detectable glycerophosphatase exists in the roots.