Increase in NAD(P)H-dependent generation of active oxygen species and changes in lipid composition of microsomes isolated from roots of zinc-deficient bean plants

The role of zinc (Zn) in maintaining the structural and functional integrit y of plant membranes was investigated in the present work. The relationship between the activity of NAD(P)H oxidases generating active oxygen species and changes in lipid composition and peroxidation was evaluated in microsom al membrane vesicles isolated from roots of Zn-deficient bean (Phaseolus vu lgaris L., cv. Bobis) plants. Zinc content of bean root microsomal membrane s was decreased by about 30% by Zn deficiency. Microsomes isolated from roo ts of Zn-deficient plants showed higher rates of NAD(P)H oxidation and NAD( P)H-dependent O-2- generation than Zn-sufficient roots. Microsomal O-2(-) c onsumption, measured in the presence of pyridine nucleotides, was also cons iderably enhanced by Zn deficiency. This latter activity was greatly stimul ated by Fe(III)EDTA, while inhibited by superoxide dismutase (SOD) and cata lase, indicating that active oxygen species were produced during the oxygen consuming enzyme reaction. Zinc deficiency caused a decline in microsomal phospholipid (PL) content. In addition, saturated fatty acids were present at a higher proportion than unsaturated fatty acids in microsomes from Zn-d eficient roots. Sterol content of microsomal vesicles was also modified by Zn deficiency, which led to an increase in the planar sterol campesterol an d a concomitant decrease in stigmasterol and sitosterol content. NADPH-depe ndent lipid peroxidation, directly measured in microsomal vesicles as malon dialdehyde (MDA) production, was slightly enhanced by Zn deficiency. These results support the idea that Zn deficiency determines an enhanced generati on of harmful oxygen species by membrane-associated enzymes and show that t his activity can be more pronounced in the presence of iron (Fe), which acc umulates in Zn-deficient tissues. The relationship between the occurrence o f this phenomenon and the changes in membrane lipid profile is discussed.