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Factors Involved in in Vitro Stabilization of Nitrate Reductase from Cotton (Gossypium hirsutum L.) Cotyledons ¹

Department of Plant Sciences, Texas A&M University, College Station, Texas 77843

Experiments were conducted to determine if pretreatment of cotton (Gossypium hirsutum L.) plants resulted in differential in vitro stabilities of nitrate reductase (NR) activity. Although NR activity declines markedly during the second half of the daily light period, in vitro NR stability is not modified by time of harvest. Phenylmethylsulfonylfluoride, iodoacetamide, and N-ethylmaleimide do not influence in vitro NR stability, suggesting that serine or sulfhydryl proteases are not responsible for in vitro lability of NR from cotton cotyledons.

Imposition of water stress or artificial extension of the dark period lead to significant reductions in NR activity, but do not change in vitro NR stability.

Dilution of a crude extract leads to increasing lability of NR; hence the marked instability of NR cannot be attributed to an inactivator which follows simple enzyme kinetics. Since in vitro NR activity is much more stable in presence of both NADH and NO₃^–, substrate availability must be considered as a possible factor influencing in vivo NR stability.

³ Present address: Texas Agricultural Experiment Station, Blackland Research Center, Box 748, Temple, Texas 76501.

¹ This research was supported in part by NDEA Title IV Fellowship 72-09441 awarded to C. R. T. A contribution of the Texas Agricultural Experiment Station.