Nitrate reductase activity in cotton cotyledons : studies on in vitro stabilization and in vivo regulation

Abstract

Cotyledons from germinating cotton seedlings exhibit an early period of nitrate reductase (NR) activity (approximately 28 hr following the initiation of imbibition) which is stable in vitro. By day 3 this activity has essentially disappeared from the cotyledons, but it is replaced (day 4 and beyond) by NR activity which is liable in vitro. Casein, bovine serum albumin (BSA), and boiled extract from 2-day old cotton cotyledons are all capable of stabilizing labile NR from older cotton cotyledons and leaves of corn, bean, and watermelon to some degree. This stabilization is not mediated by donation of a catalytic subunit to the NR molecule, but involves either donation of a structural subunit, or a general stabilization resulting from increased concentration of protein (or other factors) in solution. The three substances also "enhance" NR activity above levels of comparable buffer dilutions. This "enhancement" appears to result from the competition of casein, BSA, and the boiled extract for inhibitors initially bound to NR. After day 3, NR activity in cotton cotyledons exhibits a marked circadian variation, with maximum activity being observed approximately 6 hr into the light period. Although in vitro activity from such cotyledons is labile, the current investigation does not support an active role of proteases in modulating NR activity for three distinct reasons..