Age dependence of thyroxine stimulation of riboflavin incorporation into flavin coenzymes in liver and brain

Inhibition of flavin metabolism by adriamycin in skeletal muscle

Adriamycin (ADR), a potent antineoplastic agent, has been shown to interact with flavin derivatives and to compete with flavin coenzymes for their respective binding sites on flavin-containing enzymes. The present investigation determined that ADR administration inhibited flavin adenine dinucleotide (FAD) biosynthesis from riboflavin in rat skeletal muscle in a dose-related manner compared to results in pair-fed controls. Five groups of adult Holtzman rats of both sexes were given twice daily intraperitoneal injections of ADR for 3 days, representing cumulative doses of 6, 12, 18, 24, and 30 mg/kg body weight. At the cumulative dose of 6 mg/kg, there was no significant effect, but at 12, 18, 24, and 30 mg/kg levels significant increases in [14C]FAD formation from [14C]riboflavin occurred. ADR-induced myopathy may be due, at least in part, to inhibition of FAD formation, ultimately leading to changes in energy metabolism and oxidative capacity.

Mechanisms underlying the differential effects of ethanol on the bioavailability of riboflavin and flavin adenine dinucleotide

Chronic alcoholism is associated with a high prevalence of riboflavin deficiency. Experiments were designed in an animal model to determine whether ethanol alters selectively the absorption of riboflavin and flavin adenine dinucleotide (FAD), the predominant dietary form of the vitamin. Rats received by gavage a liver homogenate to which either [14C]riboflavin or [14C]FAD was added with either ethanol or isocaloric sucrose solutions. Ethanol markedly diminished the bioavailability of [14C]FAD to a greater degree than that of [14C]riboflavin. Corroboration of an ethanol-impaired intraluminal hydrolysis of FAD was provided by using everted jejunal segments and measuring mucosal uptake of [14C]riboflavin together with nonradiolabeled FAD. In subsequent studies with mucosal cell extracts, ethanol markedly inhibited activities of FAD pyrophosphatase and flavin mononucleotide (FMN) phosphatase. These findings suggest that dietary sources of riboflavin (FMN and FAD) are not absorbed as well in the presence of ethanol than are vitamin preparations containing riboflavin, which is utilized more readily.

Thyroid hormone regulation of flavocoenzyme biosynthesis

The means by which thyroid hormone regulates flavocoenzyme biosynthesis was studied in hyper-, eu-, and hypothyroid rats by determining the activities of flavocoenzyme-forming enzymes, viz., flavokinase and FAD synthetase, as well as those of flavocoenzyme-degrading enzymes, viz., FMN phosphatase and FAD pyrophosphatase. Flavokinase activity was increased in hyperthyroid animal and decreased in hypothyroid animals. Correspondence of flavokinase activity with the amount of a high-affinity flavin-binding protein quantitated immunologically in hypo-, eu-, and hyperthyroid rats indicated that the thyroid response is caused by an increased amount of enzyme; moreover, the concomitant decrease in a low-affinity flavin-binding protein suggests an inactive precursor form of flavokinase. FAD synthetase activity showed a similar but less pronounced trend than flavokinase. Activities of FMN phosphatase and FAD pyrophosphatase were not influenced by thyroid hormone. Overall results indicate that the mechanism of thyroid hormone regulation of flavocoenzyme level is in the steps of biosynthesis, especially at flavokinase, rather than in degradation steps.

Inhibition by chlorpromazine of thyroxine modulation of flavin metabolism in liver, cerebrum and cerebellum

In livers of adult rats that have been treated with thyroxine, the rate of incorporation of radiolabeled riboflavin into both flavin adenine dinucleotide (FAD) and FAD covalently attached to specific apoflavoenzymes was enhanced markedly. By contrast, thyroxine diminished riboflavin incorporation into FAD in cerebrum and cerebellum but continued to enhance incorporation into the covalently bound fraction of FAD. Diminished net incorporation of riboflavin into FAD in brains of adult rats may reflect increased utilization of this fraction for covalent attachment into specific apoflavoenzymes rather than down regulation of the FAD biosynthetic enzymes, flavokinase and FAD pyrophosphorylase, inasmuch as covalent attachment of FAD occurs subsequent to the formation of FAD. The psychotropic drug, chlorpromazine, over a wide dose range, exerted an inhibitory effect on both incorporation of riboflavin into FAD and the utilization of FAD for incorporation into covalently bound flavoenzymes in liver, cerebrum, and cerebellum. Thus, chlorpromazine inhibition of FAD metabolism occurred regardless of the direction of the thyroxine effect and was compatible with an observed inhibitory effect by this drug upon the flavin biosynthetic enzymes.

Aldosterone stimulation of riboflavin incorporation into rat renal flavin coenzymes and the effect of inhibition by riboflavin analogues on sodium reabsorption

This study was designed to investigate a possible relationship between the effect of aldosterone upon urinary electrolytes and the incorporation of [(14)C]riboflavin into renal [(14)C]flavin mononucleotide (FMN) and [(14)C]flavin adenine dinucleotide (FAD). Adrenalectomized Sprague-Dawley rats that weighed between 185 and 210 g were pretreated with 15 mug/100 g body wt dexamethasone intraperitoneally. 16 h later they were administered aldosterone (1.5 mug/100 g body wt) and [(14)C]riboflavin (5.0 muCi/200 g body wt). The urethra of each rat was ligated, and the rats were sacrificed by decapitation 3 h later. The urine was aspirated from the bladders of each rat and analyzed for total Na(+) and K(+) excretion while the kidneys were removed and the formation of [(14)C]FMN and [(14)C]FAD was determined for each kidney. There was a significant increase in the formation of renal [(14)C]FMN and [(14)C]FAD (27.3 and 14.4%, respectively) after aldosterone treatment. Aldosterone significantly decreased the excretion of Na(+) by 50%, and increased that of K(+) by 55%. To determine if the increased incorporation of [(14)C]riboflavin into renal [(14)C]FMN and [(14)C]FAD was an important intermediary step in the aldosterone-induced alterations in urinary Na(+) and K(+), the riboflavin analogues 7,8-dimethyl-10-formylmethyl isoalloxazine or 7,8-dimethyl-10-(2'-hydroxyethyl) isoalloxazine were given to the animals i.p. to diminish the conversion of riboflavin to FMN by competitively inhibiting the enzyme flavokinase (EC 2.7.1.26). These analogues were found to significantly counteract the decreased urinary excretion of Na(+) as a result of aldosterone from 26+/-9% to 124+/-58% (SEM) with a dose-related response when administered from 10 to 25 mug/100 g body wt. The same doses of the analogues that significantly increased the urinary output of Na(+) when administered simultaneously with aldosterone also significantly decreased the formation of renal [(14)C]FMN from 15+/-4 to 38+/-3% when compared with the effects of aldosterone alone. The analogues exerted no significant effect on the increased urinary excretion of K(+) by aldosterone. The analogues alone had no influence on urinary Na(+) and K(+) output or the formation of renal [(14)C]FMN and [(14)C]FAD at the dose levels that we investigated. These data strongly suggest that the enhanced synthesis of renal FMN and FAD may be a causative factor in the increased reabsorption of Na(+) as a result of aldosterone; and, consequently, riboflavin analogues may function as a novel class of antimineralocorticoids.