In vitro alleviation of heavy metal-induced enzyme inhibition by proline

Accumulation of proline in response to toxic heavy metal exposure seems to be wide-spread among plants. To elucidate the role for proline in plant res ponses to heavy metal stress, we studied the effect of proline on Cd-induce d and Zn-induced inhibition of glucose-6-phosphate dehydrogenase (G-6-PDH; EC 1.1.1.49) and nitrate reductase (NR; EC 1.6.6.2) in vitro. Proline appea red to protect both enzymes against Zn and, though less effectively, agains t Cd. Measurements with a Cd2+-specific electrode strongly suggested that t his protection was based on a reduction of the free metal ion activity in t he assay buffer, due to the formation of metal-proline complexes. There wer e no indications of any significant role for proline-water or proline-prote in interactions. The significance of these findings with regard to heavy me tal-induced proline accumulation in viva is discussed. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.