THE ISOMERIZATION OF GLUTAMATE-DEHYDROGENASE IN RESPONSE TO LEAD TOXICITY IN MAIZE

Maize (Zea mays) was cultivated on lead-adultrated soil up to 600 mg(P b) kg(-1). At maturity, the maize seeds were harvested. The glutamate dehydrogenase (GDH) was fractionated to its isoenzyme population by Ro tofor isoelectric focusing (IEF). The increasing Pb concentration prog ressively enhanced the more acidic isoenzymes (pI 6.3 - 6.5), and at t he same time suppressed the less acidic isoenzymes (pI 7.3 - 7.8) and at the 600 mg(Pb) kg(-1)(soil) only the most acidic couple of isoenzym es (pi 6.3, and 6.5) were detectable. The NH4+ Km values of the GDH in creased progressively from 6.2 in the control to 100 mM and the total glutathione content of maize seeds from 60 to 240 nmol g(-1) in the 60 0 mg(Pb) kg(-1)(soil) treated maize. The orderly, and sequential isome rization of GDH in response to Pb suggests that the enzyme functions a s a sensor in the monitoring of environmentally induced stress.