Uptake of Lead and Cadmium by Maize Seedlings and the Effect of Heavy Metals on the Activity of Phosphoenolpyruvate Carboxylase Isolated from Maize

Abstract

Maize seeds and five-day-old maize seedlings were incubated in media containing Pb²⁺ at concentrations of 50, 100, and 200 mg 1^-1 and Cd²⁺ at concentrations of 1, 5, 10 and 50 mg 1^-1. After five days of incubation, both heavy metals were determined by means of AAS following wet mineralisation of roots and shoots. The results obtained indicate that Pb²⁺ were transported to shoots from roots at a lower rate than Cd²⁺.

Phosphoenolpyruvate carboxylase (PEPC) isolated from germinating maize seeds was inhibited to a comparable degree by solutions containing 0.001 mmol 1^-1 Pb²⁺, 0.01 mmol 1^-1 Cd²⁺, and 0.005 mmol 1^-1 Cu²⁺. The enzyme was protected against this inhibition by the addition of mercaptoethanol, the substrate (PEP), or the cofactor (Mg²⁺). The inhibition increased during a 20 min incubation of the enzyme with salts of the metals. Mn²⁺, Ni²⁺, and Co²⁺ ions could partially substitute for the metal cofactor Mg²⁺. K_m values for these metal ions were as follows: for Mg²⁺ 0.07 mmol 1^-1 in the range from 0 to 0.30 mmol 1^-1 Mg²⁺; 0.71 mmol 1^-1 for 0.30 to 2.50 mmol 1^-1 Mg²⁺; for Mn²⁺ 0.36 mmol 1^-1; for Ni²⁺ 0.34 mmol 1{-1}; and for Co²⁺ 0.20 mmol 1^-1. The activity of the enzyme reached with Mn²⁺ 85 %, with Ni²⁺ 65 %, and with Co²⁺ 55 % of the activity recorded with Mg²⁺.