Induction of flavokinase (EC 2.7.1.26) by aldosterone in the rat kidney

The effect of aldosterone on rat renal flavokinase (EC 2.7.1.26) enzymic activity and concentration was investigated in bilaterally adrenalectomized male Sprague-Dawley rats. Flavokinase enzymic activity was measured in the 100,000 X g supernatant of renal cortex and red medulla and was increased after 3 h by 19% and 42%, respectively, as a result of aldosterone (1.5 micrograms/100 g BW) administration. Dual isotope labeling studies revealed increases of 20-30% and 20-35% in the incorporation of [3H]- and [35S]methionine into rat renal cortical and red medullary flavokinase, respectively, as a result of aldosterone administration. The aldosterone-dependent increases in methionine incorporation were blocked when the mineralocorticoid receptor antagonist spirolactone (SC 26304; 150 micrograms/100 BW) was administered 30 min before aldosterone. The relative concentrations of renal flavokinase also increased by 40% in both the cortex and red medulla 3 h after aldosterone treatment, as determined by an enzyme-linked immunosorbent assay (ELISA). These increases were abolished by actinomycin D (100 micrograms/100 g BW) and cycloheximide (200 micrograms/100 g BW), given 1 h before aldosterone administration. The mineralocorticoid receptor antagonists SC 26304 (225 micrograms/100 g BW) and progesterone (500 micrograms/100 g BW) were also able to inhibit the aldosterone-dependent increase in renal flavokinase concentration. The effect of aldosterone on renal flavokinase concentration was studied from 30 min to 8 h after aldosterone administration. There appeared to be maximum increases of 23-30% and 25-32% after 2.5-3.5 h in the renal cortex and red medulla, respectively. 5 alpha-Dihydroaldosterone, a metabolite of aldosterone with mineralocorticoid activity, was also able to increase renal flavokinase concentrations by approximately 40%. However, dexamethasone, a potent glucocorticoid with little or no mineralocorticoid activity, appeared to have no effect on renal flavokinase, as observed by ELISA. These data suggest that the increase in the relative concentration of renal flavokinase may be due to increased biosynthesis of flavokinase, and ultimately, that renal flavokinase may be an aldosterone-induced protein whose synthesis is mediated through the mineralocorticoid receptor and RNA synthesis.