Determination of kinetic parameters of acid phosphatases in intact sugar beet roots of variable phosphorus nutrition

Organically bound phosphorus has to be hydrolysed before its P can be taken up by plants. Both microbes and plant roots possess phosphatases, which co uld be of importance especially in soils with low concentrations of inorgan ic phosphorus in the soil solution. This could be the reason why nutrient u ptake models underestimate the P-uptake by plants when P-mobilization by th e phosphatases of roots is not taken into consideration. Therefore the acti vity of acid phosphatases (P-ase) was determined to answer the following qu estions: 1) To which extent does the root bound acid phosphatase (P-ase) fo llow Michaelis-Menten kinetics? 2) By which of the four linear transformati ons of the Michaelis-Menten equation (Line-weaver/Burk, Hanes, Eadie/Hofste e, Eisenthal/Cornish-Bow;den) can plausible values of V-max and K-m be dete rmined? 3) Which effect has the P nutrition of the plant on these kinetic p arameters? Sugar beet plants were grown in full nutrient solution containin g I and 100 mu M P respectively. The P-ase activity of the intact roots was measured at pH 5 using p-nitrophenylphosphate (25-15000 mu M p-NPP). V-max values were calculated per m root length. Acid phosphatase activity princi pally followed Michaelis-Menten kinetics. Transformations and calculations of V-max and K-m after Eadie/Hofstee and Eisenthal/Cornish-Bowden suggested the existence of at least two enzyme systems (P-ase 1, P-ase 2) The follow ing kinetic parameters were found: P-ase 1: P deficient plants: V-max: 43-45 nmol m(-1) min(-1), K-m: 31-37 mu M NPP; P sufficient plants: V-max: 7 nmol m(-1) min(-1), K-m: 47-53 mu M NPP. P-ase 2: P deficient plants: V-max: 230-293 nmol m(-1) min(-1), K-m: 1579-3845 mu M NPP; P sufficient plants: V-max: 123-171 nmol m(-1) min(-1), K-m: 3027-7000 mu M NPP. Thus plants with sufficient P nutrition have a lower affinity to P-org and a lower hydrolysis of P-org For P nutrition of crops P-ase 1 might be the m ost important enzyme.